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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

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SCHEDULE TO

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)
of the Securities Exchange Act of 1934

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WHITE MOUNTAINS INSURANCE GROUP, LTD.
(Name of Subject Company (Issuer) and Filing Person (Issuer and Offeror))

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Common Shares, par value $1.00 per share
(Title of Class of Securities)

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G9618E107
(CUSIP Number of Class of Securities)

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Robert L. Seelig, Esq.
Executive Vice President and General Counsel
White Mountains Insurance Group, Ltd.
80 South Main Street
Hanover, New Hampshire 03755-2053
Telephone: (603) 640-2200
(Name, address and telephone number of person authorized to receive notices and communications
on behalf of filing persons)

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With a copy to:

Philip A. Gelston, Esq.
Cravath, Swaine & Moore LLP
825 Eighth Avenue
New York, New York 10019
Telephone: (212) 474-1000

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CALCULATION OF FILING FEE:

o

Check the box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

o

Check the box if the filing relates solely to preliminary communications made before the commencement of the tender offer.

Check the appropriate boxes below to designate any transactions to which the statement relates:

o

third-party tender offer subject to Rule 14d-1.



ý

issuer tender offer subject to Rule 13e-4.



o

going-private transaction subject to Rule 13e-3.



o

amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results of the tender offer:    o

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

o

Rule 13e-4(i) (Cross-Border Issuer Tender Offer)



o

Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)

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INTRODUCTION

        This Issuer Tender Offer Statement on Schedule TO ("Schedule TO") relates to the tender offer by White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), to purchase 500,000 of its issued and outstanding Common Shares, par value $1.00 per share (the "Common Shares" or the "Shares"), or such lesser number of Shares as is properly tendered and not properly withdrawn, at a price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase dated August 17, 2017 (the "Offer to Purchase"), a copy of which is attached hereto as Exhibit (a)(1)(A), and in the related Letter of Transmittal (the "Letter of Transmittal" and, together with the Offer to Purchase, as they may be amended or supplemented from time to time, the "Offer"), a copy of which is attached hereto as Exhibit (a)(1)(B), which are herein incorporated by reference. This Schedule TO is being filed in accordance with Rule 13e—4 under the Securities Exchange Act of 1934, as amended (the "Exchange Act").

        All information in the Offer is hereby expressly incorporated by reference in answer to all items in this Schedule TO, and as more particularly set forth below.

Item 1.    Summary Term Sheet.

        The information set forth in the Offer to Purchase under "Summary Term Sheet" is incorporated herein by reference.

Item 2.    Subject Company Information.

        (a)   The name of the issuer is White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda. The Company's principal executive offices are located at 80 South Main Street, Hanover, New Hampshire 03755-2053, telephone: (603) 640-2200.

        (b)   The class of securities to which this statement relates is the Common Shares, par value $1.00 per share, of which 4,336,625 Shares were issued and outstanding as of August 16, 2017.

        (c)   The information set forth in the Offer to Purchase under Section 7 ("Price Range of Shares; Dividends") is incorporated herein by reference.

Item 3.    Identity and Background of Filing Person.

        (a)   This Tender Offer Statement on Schedule TO is filed by the Company, which is also the issuer. The Company's address and telephone number are set forth under Item 2. The information set forth in the Offer to Purchase under Schedule I and Section 11 ("Interests of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares") is incorporated herein by reference.

Item 4.    Terms of the Transaction.

        (a)   The following information set forth in the Offer to Purchase is incorporated herein by reference:

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"Summary Term Sheet";



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"Introduction";



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Section 1 ("Number of Shares; Proration");



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Section 2 ("Procedures for Tendering Shares");



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Section 3 ("Withdrawal Rights");



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Section 4 ("Acceptance for Payment and Payment for Shares");



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Section 5 ("Purpose of the Offer; Certain Effects of the Offer");



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Section 6 ("Certain U.S. Federal Income Tax Consequences");

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•

Section 8 ("Conditions to the Offer");



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Section 11 ("Interest of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares"); and



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Section 14 ("Extension of the Offer; Termination; Amendment").

        (b)   The information set forth in the Offer to Purchase under "Summary Term Sheet", "Introduction", Section 5 ("Purpose of the Offer; Certain Effects of the Offer") and Section 11 ("Interest of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares") is incorporated herein by reference.

Item 5.    Past Contacts, Transactions, Negotiations and Agreements.

        (e)   The information set forth in the Offer to Purchase under Section 11 ("Interests of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares") is incorporated herein by reference.

Item 6.    Purpose of the Transaction and Plans or Proposals.

        (a)   The information set forth in the Offer to Purchase under "Summary Term Sheet" and Section 5 ("Purpose of the Offer; Certain Effects of the Offer") is incorporated herein by reference.

        (b)   The information set forth in the Offer to Purchase under Section 5 ("Purpose of the Offer; Certain Effects of the Offer") is incorporated herein by reference.

        (c)   The information set forth in the Offer to Purchase under "Summary Term Sheet", "Introduction", Section 5 ("Purpose of the Offer; Certain Effects of the Offer"), Section 9 ("Certain Information Concerning the Company") and Section 10 ("Source and Amount of Funds") is incorporated herein by reference.

Item 7.    Source and Amount of Funds or Other Consideration.

        (a), (b) and (d) The information set forth in the Offer to Purchase under "Summary Term Sheet" and Section 10 ("Source and Amount of Funds") is incorporated herein by reference.

Item 8.    Interest in Securities of the Subject Company.

        (a)   and (b) The information set forth in the Offer to Purchase under Schedule I and Section 11 ("Interests of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares") is incorporated herein by reference.

Item 9.    Persons/Assets Retained, Employed, Compensated or Used.

        (a)   The information set forth in the Offer to Purchase under "Introduction" and Section 15 ("Fees and Expenses") is incorporated herein by reference.

Item 10.    Financial Statements.

        Not applicable.

Item 11.    Additional Information.

        (a)(1)  The information set forth in the Offer to Purchase under Section 11 ("Interests of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares") is incorporated herein by reference.

        (a)(2)  The information set forth in the Offer to Purchase under Section 12 ("Certain Legal Matters") is incorporated herein by reference.

        (a)(3)  Not applicable.

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        (a)(4)  The information set forth in the Offer to Purchase under Section 13 ("Effects of the Offer on the Market for Shares; Registration Under the Exchange Act") is incorporated herein by reference.

        (a)(5)  None.

        (b)   The information set forth in the Offer to Purchase and the Letter of Transmittal, as each may be amended or supplemented from time to time, is incorporated herein by reference.

Item 12.    Exhibits.

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Item 13.    Information Required by Schedule 13E-3.

        Not applicable.

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SIGNATURE

        After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

August 17, 2017

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INDEX OF EXHIBITS

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INTRODUCTION

Item 1. Summary Term Sheet.
Item 2. Subject Company Information.
Item 3. Identity and Background of Filing Person.
Item 4. Terms of the Transaction.
Item 5. Past Contacts, Transactions, Negotiations and Agreements.
Item 6. Purpose of the Transaction and Plans or Proposals.
Item 7. Source and Amount of Funds or Other Consideration.
Item 8. Interest in Securities of the Subject Company.
Item 9. Persons/Assets Retained, Employed, Compensated or Used.
Item 10. Financial Statements.
Item 11. Additional Information.
Item 12. Exhibits.
Item 13. Information Required by Schedule 13E-3.

SIGNATURE
INDEX OF EXHIBITS

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TABLE OF CONTENTS

Exhibit (a)(1)(A)

OFFER TO PURCHASE FOR CASH
BY
WHITE MOUNTAINS INSURANCE GROUP, LTD.
OF UP TO 500,000 OF ITS COMMON SHARES
AT A PURCHASE PRICE NOT GREATER THAN $875
OR LESS THAN $825 PER SHARE

        White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company", "White Mountains", "we" or "us"), is offering to purchase 500,000 of its common shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in this Offer to Purchase (as it may be amended or supplemented from time to time, this "Offer to Purchase") and in the related Letter of Transmittal (which together, as they may be amended or supplemented from time to time, constitute the "Offer"), including the "odd lot" and proration provisions described herein. We will select the lowest price per Share (in increments of $1.00) (the "Purchase Price") that will allow us to purchase the number of Shares sought in the Offer or, if a lesser number of Shares is properly tendered, all Shares that are properly tendered and not properly withdrawn. All Shares acquired in the Offer will be acquired at the same price regardless of whether the shareholder tendered at a lower price. However, because of the proration and "odd lot" priority provisions described in this Offer to Purchase, fewer than all of the Shares tendered at or below the Purchase Price may be purchased if more than the number of Shares the Company seeks are properly tendered and not properly withdrawn. Shares tendered but not purchased in the Offer, including Shares tendered at or below the Purchase Price and Shares not purchased because of proration, will be returned to the tendering shareholders at the Company's expense promptly after the expiration of the Offer. See "Section 1. Number of Shares; Proration" and "Section 2. Procedures for Tendering Shares".

        In the event that more than 500,000 Shares are properly tendered in the Offer, we may exercise our right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of our outstanding Shares without extending the Expiration Time. We also expressly reserve the right, in our sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law. See "Section 1. Number of Shares; Proration" and "Section 14. Extension of the Offer; Termination; Amendment".

        This Offer is not conditioned upon the receipt of financing or any minimum number of Shares being tendered. This Offer is, however, subject to certain other conditions. See "Section 8. Conditions to the Offer".

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        The Shares are listed and traded on the New York Stock Exchange (the "NYSE") under the symbol "WTM" and the Bermuda Stock Exchange under the symbol "WTM-BH". On August 16, 2017, the last full trading day before the public announcement of the Offer, the last reported closing sale price of the Shares on the NYSE was $850.65 per Share. SHAREHOLDERS ARE URGED TO OBTAIN A CURRENT MARKET QUOTATION FOR THE SHARES. See "Section 7. Price Range of Shares; Dividends".

        OUR BOARD OF DIRECTORS (THE "BOARD") HAS APPROVED THE OFFER. HOWEVER, NONE OF THE COMPANY, OUR BOARD, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT MAKES ANY RECOMMENDATION AS TO WHETHER ANY SHAREHOLDER SHOULD TENDER OR REFRAIN FROM TENDERING SHARES PURSUANT TO THE OFFER, OR AS TO THE PRICE OR PRICES AT WHICH A SHAREHOLDER MAY CHOOSE TO TENDER SHARES. EACH SHAREHOLDER MUST MAKE HIS OR HER OWN DECISION AFTER CONSULTING WITH HIS OR HER OWN ADVISORS WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO TENDER AND THE PRICE OR PRICES AT WHICH SUCH SHAREHOLDER WILL TENDER THEM. IN DOING SO, SHAREHOLDERS SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE AND IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING THE PURPOSES AND EFFECTS OF THE OFFER. SEE "SECTION 5. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER".

        ALL OF OUR DIRECTORS AND EXECUTIVE OFFICERS HAVE ADVISED US THAT THEY DO NOT INTEND TO TENDER ANY SHARES OWNED BY THEM IN THE OFFER. THE TRUST UNDER THE EXECUTIVE SURVIVOR INCOME PLAN MAINTAINED BY OUR SUBSIDIARY, ONEBEACON SERVICES, LLC, MAY ELECT TO TENDER A NUMBER OF SHARES REPRESENTING UP TO AN AGGREGATE VALUE OF $4.0 MILLION. THERE CAN BE NO ASSURANCES THAT SUCH TRUST WILL TENDER SHARES IN THE OFFER OR THE PRICE OR PRICES AT WHICH ANY SUCH SHARES MAY BE TENDERED. SEE "SECTION 11. INTERESTS OF DIRECTORS AND EXECUTIVE OFFICER; TRANSACTIONS AND ARRANGEMENTS CONCERNING THE SHARES".

        NEITHER THE SECURITIES AND EXCHANGE COMMISSION ("SEC") NOR ANY STATE SECURITIES COMMISSION HAS APPROVED OR DISAPPROVED OF THIS TRANSACTION OR PASSED UPON THE FAIRNESS OR MERITS OF SUCH TRANSACTION OR UPON THE ACCURACY OR ADEQUACY OF THIS DOCUMENT. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

        You may direct questions and requests for assistance to D.F. King & Co., Inc., which is acting as the Information Agent for the Offer ("D.F. King" or the "Information Agent"), or to Barclays Capital Inc. or J.P. Morgan Securities LLC, which are severally acting as the Dealer Managers for the Offer (the "Dealer Managers"). Their respective addresses and telephone numbers appear on the back cover of this Offer to Purchase. You may direct requests for additional copies of this Offer to Purchase, the Letter of Transmittal or the Notice of Guaranteed Delivery to the Information Agent.

The Dealer Managers for the Offer are:

Offer to Purchase dated August 17, 2017

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IMPORTANT

        Any shareholder desiring to tender all or any portion of such shareholder's Shares should (1) if such shareholder holds the Shares in its own name, complete and sign the Letter of Transmittal in accordance with the instructions in the Letter of Transmittal, have such shareholder's signature thereon guaranteed (if required by Instruction 1 to the Letter of Transmittal), mail, express or overnight delivery, or deliver the Letter of Transmittal and any other required documents to the Depositary (as defined herein) and either deliver the certificates for such Shares (the "Certificates") along with the Letter of Transmittal to the Depositary or tender such Shares pursuant to the procedures for book-entry transfer set forth in "Section 2. Procedures for Tendering Shares", (2) if such shareholder's Shares are registered in the name of a broker, dealer, commercial bank, trust company or other nominee, request such shareholder's broker, dealer, commercial bank, trust company or other nominee to effect the transaction for such shareholder or (3) if such shareholder is an institution participating in The Depository Trust Company, tender the Shares according to the procedure for book-entry transfer described in "Section 2. Procedures for Tendering Shares". Any shareholder whose Shares are registered in the name of a broker, dealer, commercial bank, trust company or other nominee must contact such broker, dealer, commercial bank, trust company or other nominee to tender such Shares.

        Any shareholder who desires to tender Shares and whose Certificates are not immediately available, or who cannot comply with the procedures for book-entry transfer described in this Offer to Purchase on a timely basis or who cannot deliver all required documents to the Depositary prior to the expiration of the Offer, may nevertheless tender such Shares by following the procedures for guaranteed delivery set forth in "Section 2. Procedures for Tendering Shares".

        If you wish to maximize the chance that your Shares will be purchased by us, you should check the box in the section of the Letter of Transmittal captioned "Shares Tendered at a Price Determined Pursuant to the Offer". If you agree to accept the Purchase Price determined pursuant to the Offer, your Shares will be deemed to be tendered at the minimum price of $825 per share. You should understand that this election may lower the Purchase Price paid for all purchased Shares in the Offer and could result in your Shares being purchased at the minimum price of $825 per Share, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.

        TO TENDER SHARES PROPERLY, YOU OR, IN THE CASE OF SHARES REGISTERED IN THE NAME OF A BROKER, DEALER, COMMERCIAL BANK, TRUST COMPANY OR OTHER NOMINEE, YOUR NOMINEE MUST PROPERLY COMPLETE AND DULY EXECUTE THE LETTER OF TRANSMITTAL OR AN AGENT'S MESSAGE IN LIEU OF THE LETTER OF TRANSMITTAL, INCLUDING THE SECTION RELATING TO THE PRICE AT WHICH SHARES ARE BEING TENDERED.

        THE COMPANY IS NOT MAKING THE OFFER TO, AND WILL NOT ACCEPT ANY TENDERED SHARES FROM, SHAREHOLDERS IN ANY U.S. STATE WHERE IT WOULD BE ILLEGAL TO DO SO. HOWEVER, THE COMPANY MAY, IN ITS DISCRETION, TAKE ANY ACTIONS NECESSARY TO MAKE THE OFFER TO SHAREHOLDERS IN ANY SUCH STATE.

        Questions and requests for assistance may be directed to the Information Agent or to either of the Dealer Managers at their respective addresses and telephone numbers set forth on the back cover of this Offer to Purchase. Additional copies of this Offer to Purchase, the Letter of Transmittal, the Notice of Guaranteed Delivery and other tender offer materials may also be obtained from the Information Agent or either of the Dealer Managers at the Company's expense. Shareholders may also contact their brokers, dealers, commercial banks or trust companies for assistance concerning the Offer.

        NO PERSON HAS BEEN AUTHORIZED TO MAKE ANY RECOMMENDATION ON BEHALF OF THE COMPANY AS TO WHETHER SHAREHOLDERS SHOULD TENDER OR REFRAIN FROM TENDERING SHARES PURSUANT TO THE OFFER, OR AS TO THE PRICE OR PRICES AT WHICH A SHAREHOLDER MAY CHOOSE TO TENDER SHARES. NO PERSON HAS BEEN AUTHORIZED TO GIVE ANY INFORMATION OR TO MAKE ANY REPRESENTATIONS IN

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CONNECTION WITH THE OFFER OTHER THAN THOSE CONTAINED OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE OR IN THE LETTER OF TRANSMITTAL. IF MADE OR GIVEN, SUCH RECOMMENDATION AND SUCH INFORMATION AND REPRESENTATIONS MUST NOT BE RELIED UPON AS HAVING BEEN AUTHORIZED BY THE COMPANY, THE BOARD, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT.

        Rule 13e-4 under the Securities Exchange Act of 1934, as amended (the "Exchange Act") generally prohibits us and our affiliates from purchasing any Shares, other than pursuant to the Offer, during the Offer and for the period ending ten business days after the expiration or termination of the Offer. Following that time, we expressly reserve the absolute right, in our sole discretion from time to time in the future, to purchase Shares, whether or not any Shares are purchased pursuant to the Offer, through open market purchases, privately negotiated transactions, tender offers, exchange offers or otherwise, upon the same terms or on terms that are more or less favorable to the selling shareholders in those transactions than the terms of the Offer. We cannot assure you as to which, if any, of these alternatives, or combinations thereof, we might pursue.

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TABLE OF CONTENTS

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SUMMARY TERM SHEET

        White Mountains Insurance Group, Ltd. (the "Company", "White Mountains", "we" or "us"), is offering to purchase up to 500,000 of its common shares, par value $1.00 per share (the "Shares"), at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions as set forth in this Offer to Purchase. We are providing this summary term sheet containing frequently asked questions and responses for your convenience. We urge you to read this Offer to Purchase and Letter of Transmittal and the other documents that are incorporated by reference herein because the information in this summary is not complete. We have included references to the sections of this Offer to Purchase where you will find a more complete discussion of the topics in this summary.

Who Is Offering To Buy My Securities, And Why?

        The Offer to purchase the Shares is being made by White Mountains, a company organized under the laws of Bermuda. The Offer is being made to utilize a portion of the Company's undeployed capital and to provide shareholders with added liquidity. If you choose to tender your Shares, you will avoid the usual transaction costs associated with any market sale. If you choose not to tender your Shares, the Offer will serve to increase your ownership interest in the Company and thus in the Company's future earnings and assets, because the Shares purchased pursuant to the Offer will be cancelled. See "Section 5. Purpose of the Offer; Certain Effects of the Offer" and "Section 9. Certain Information Concerning the Company".

What Are The Classes And Amounts Of Securities Sought In The Offer?

        We are offering to purchase 500,000 Shares, or such lesser number of Shares as are properly tendered and not properly withdrawn, upon the terms and subject to the conditions set forth in this Offer to Purchase and in the related Letter of Transmittal, including the "odd lot" and proration provisions described in "Section 1. Number of Shares; Proration". As of August 16, 2017, there were 4,336,625 Shares outstanding. The Shares we are seeking to purchase through this Offer represent approximately 12% of the Shares outstanding as of such date. See "INTRODUCTION".

        The Shares are currently "margin securities" under the rules of the Federal Reserve Board. This has the effect, among other things, of allowing brokers to extend credit on the collateral of the Shares. Following the repurchase of Shares pursuant to the Offer, the Shares not purchased will continue to be "margin securities" for purposes of the Federal Reserve Board's margin regulations. See "Section 13. Effects of the Offer on the Market for Shares; Registration under the Exchange Act".

        If more than 500,000 Shares are properly tendered in the Offer, we may exercise our right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of our outstanding Shares without extending the Expiration Time. We also expressly reserve the right, in our sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law. See "Section 1. Number of Shares; Proration".

        If more than 500,000 Shares (or such greater number of Shares as we may elect to purchase, subject to applicable law) are properly tendered, all Shares properly tendered at or below the Purchase Price will be purchased on a pro rata basis, subject to the odd lot priority provisions described herein. The Offer is not conditioned on any minimum number of Shares being tendered by shareholders but is subject to certain other conditions. See "Section 1. Number of Shares; Proration" and "Section 8. Conditions to the Offer".

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How Much Are You Offering To Pay And What Is The Form Of Payment?

        We are conducting this Offer through a procedure called a "modified Dutch auction". This procedure allows you to select the price (in increments of $1.00) within the price range specified by us at which you are willing to sell your Shares. The price range for the Offer is $825 to $875 per Share. See "Section 1. Number of Shares; Proration".

        We will select the lowest purchase price per Share (in increments of $1.00) (the "Purchase Price") that will allow us to buy the number of Shares sought in the Offer or, if a lesser number of Shares is properly tendered, all Shares that are properly tendered and not properly withdrawn. The lower end of the price range for the Offer is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65. We will determine the Purchase Price promptly after the Expiration Time. For purposes of determining the Purchase Price, those Shares that are tendered by shareholders agreeing to accept the Purchase Price determined pursuant to the Offer, as described below, will be deemed to be tendered at the minimum price of $825. See "Section 1. Number of Shares; Proration".

        All Shares that we purchase will be purchased at the Purchase Price, even if you have selected a lower price, but we will not purchase in the Offer any Shares tendered at a price above the Purchase Price we determine. If you wish to maximize the chance that your Shares will be purchased, you should check the box in the section on the Letter of Transmittal, "Shares Tendered at a Price Determined Pursuant to the Offer", indicating that you will accept the Purchase Price we determine. If you agree to accept the Purchase Price determined pursuant to the Offer, your Shares will be deemed to be tendered at the minimum price of $825 per Share. You should understand that this election could result in your Shares being purchased at the minimum price of $825 per Share. See "Section 1. Number of Shares; Proration" and "Section 2. Procedures for Tendering Shares".

        If your Shares are purchased in the Offer, you will receive the Purchase Price, net to the seller in cash, less any applicable withholding taxes and without interest, promptly after the Expiration Time. Under no circumstances will we pay interest on the Purchase Price, including, but not limited to, by reason of any delay in making payment. The Offer is scheduled to expire at 12:00 midnight, New York City time, at the end of the day on September 14, 2017, unless the Offer is extended by us. See "Section 1. Number of Shares; Proration" and "Section 14. Extension of the Offer; Termination; Amendment".

What Will Happen If The Offer Is Undersubscribed?

        In the event that fewer than the number of Shares we seek are properly tendered and not properly withdrawn, subject to the terms and conditions of this Offer, we will purchase all such tendered Shares.

What Happens If More Than 500,000 Shares Are Tendered At Or Below The Purchase Price? Will Tendered Shares Be Prorated?

        In the event that more than 500,000 Shares (or such greater number of Shares as we may elect to purchase, subject to applicable law) are properly tendered at or below the Purchase Price and not properly withdrawn, we will accept Shares for purchase in the following order of priority:

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First, we will purchase all "odd lots" of less than 100 Shares at the Purchase Price from shareholders who properly tender all of their Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time; and



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Second, after purchasing all the "odd lots" that were properly tendered at or below the Purchase Price, we will purchase Shares at the Purchase Price from all other holders who properly tender Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time, on a pro rata basis, with appropriate adjustments to avoid purchases of fractional Shares, until we have acquired the number of Shares that we have offered to purchase.

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        Therefore, we may not purchase all of the Shares that you tender even if you tender them at or below the Purchase Price. See "Section 1. Number of Shares; Proration".

If I Own Fewer Than 100 Shares And I Tender All Of My Shares, Will I Be Subject To Proration?

        If you own beneficially or of record fewer than 100 Shares in the aggregate, properly tender all of your Shares at or below the Purchase Price and do not properly withdraw them before the Expiration Time, and complete the section entitled Odd Lots in the Letter of Transmittal and, if applicable, in the Notice of Guaranteed Delivery, we will purchase all of your Shares without subjecting them to the proration procedure. See "Section 1. Number of Shares; Proration".

Do You Have The Financial Resources To Make Payment?

        If we purchase 500,000 Shares, and the Purchase Price is an amount between $825 and $875 per Share, then the aggregate cost to us will be between approximately $412.5 million and $437.5 million. We anticipate meeting this cost with available cash, including cash from the sale of liquid investments. We may also draw on our existing revolving credit facility to fund the purchase of Shares.

        The Offer is not subject to any financing condition. See "Section 10. Source and Amount of Funds".

Are There Any Conditions To The Offer?

        Our obligation to purchase Shares tendered depends upon a number of conditions that must be satisfied or waived by us prior to the Expiration Time. See "Section 8. Conditions to the Offer".

How Do I Tender My Shares?

        If you own your Shares in your own name as a holder of record and decide to tender your Shares, you must deliver the Certificates evidencing your Shares, together with a completed Letter of Transmittal with any required signature guarantees or other required documents, to Computershare Trust Company, N.A., the depositary for the Offer ("Computershare" or the "Depositary"), not later than the Expiration Time. Deliveries to the Company, the Information Agent, the Dealer Managers or The Depository Trust Company ("DTC") will not be forwarded to the Depositary and will not constitute a valid delivery.

        If your Shares are registered in the name of a broker, dealer, commercial bank, trust company or other nominee, meaning your Shares are owned in "street name", contact your broker, dealer, commercial bank, trust company or other nominee and have it tender your Shares for you.

        If you are unable to deliver something that is required to the Depositary by the Expiration Time, you may get extra time to do so by having a broker, bank or other fiduciary who is a member of the Securities Transfer Agent Medallion Program, The New York Stock Exchange Medallion Program or the Stock Exchange Medallion Program guarantee that the missing items will be received by the Depositary within two business days. However, the Depositary must receive the missing items within that two-business-day trading period. See "Section 2. Procedure for Tendering Shares".

        If you are a participant in the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (the "OneBeacon 401(k) Plan"), and you wish to tender any of your Shares held in the OneBeacon 401(k) Plan, you must follow the separate instructions and procedures described in Section 2 (including an earlier deadline for delivering materials) and you must review separate instructions related to the OneBeacon 401(k) Plan enclosed with this Offer to Purchase.

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How Long Do I Have To Tender In The Offer?

        You will have until 12:00 midnight, New York City time, at the end of the day on September 14, 2017 , to tender your Shares in the Offer, unless we extend the Offer. We may choose to extend the Offer in our sole discretion at any time. Further, if you cannot deliver everything that is required in order to make a valid tender by that time, you may be able to use a guaranteed delivery procedure, which is described later in this Offer to Purchase. See "Section 2. Procedures for Tendering Shares".

        If a broker, dealer, commercial bank, trust company or other nominee holds your Shares, it is likely that it has an earlier deadline for instructing it to accept the Offer. See "Section 2. Procedures for Tendering Shares".

        If you wish to tender Shares held in the OneBeacon 401(k) Plan, the deadline for giving instructions to the trustee of the OneBeacon 401(k) Plan to permit your Shares to be tendered on a timely basis is 5:00 p.m., New York City time, on September 12, 2017, unless we extend the Offer, in which case you will be able to give instructions to the trustee to tender Shares held in such Plan until 5:00 p.m., New York City time, on the date that is two business days before the new Expiration Time. See "Section 2. Procedures for Tendering Shares".

Can The Offer Be Extended Or Terminated And Under What Circumstances?

        We are reserving the right to extend the Offer in our sole discretion. Also, should we, pursuant to the terms and conditions of the Offer, change the price of the Offer, reduce the number of Shares sought in the Offer, increase the number of Shares sought in the Offer by an amount exceeding 2% of our outstanding Shares or otherwise materially amend the Offer, we will ensure that the Offer remains open long enough to comply with U.S. federal securities laws. It is possible that this could involve an extension of the Offer, which could last up to 10 additional business days in some cases. We cannot assure you, however, that we will extend the Offer or, if we extend it, for how long. See "Section 14. Extension of the Offer; Termination; Amendment".

        We also expressly reserve the right, in our sole discretion, to withdraw or terminate the Offer and not accept for payment or pay for any Shares not theretofore accepted for payment or paid for or, subject to applicable law, to postpone payment for Shares upon the occurrence of any of the conditions specified in Section 8 by giving oral or written notice of the termination or postponement to the Depositary and making a public announcement of the termination or postponement. Additionally, we expressly reserve the right, subject to applicable law, to postpone payment for Shares under circumstances including but not limited to the occurrence of any of the conditions specified in "Section 8. Conditions to the Offer" by notifying the Depositary and making a public announcement thereof.

How Will I Be Notified If The Offer Is Extended Or Amended?

        If we extend the Offer, the Company or D.F. King, the Information Agent, will make a public announcement of the extension, not later than 9:00 a.m., New York City time, on the next business day after the day on which the Offer was scheduled to expire. We will announce any amendment by making a public announcement of the amendment. See "Section 14. Extension of the Offer; Termination; Amendment".

When And How Will The Company Pay For The Shares I Tender That Are Accepted For Purchase?

        We will pay the Purchase Price, net to the seller in cash, less any applicable withholding taxes and without interest, for the Shares we purchase promptly after the expiration of the Offer and the acceptance of the Shares for payment. We will pay for the Shares accepted for purchase by depositing the aggregate Purchase Price with the Depositary promptly after the expiration of the Offer. The Depositary will act as your agent and will transmit to you the payment for all of your Shares accepted for payment. See "Section 4. Acceptance for Payment and Payment of Shares".

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May Holders Of Equity Awards Under White Mountains's Long-Term Incentive Plan Participate In The Offer?

        Holders of stock options, restricted share and performance share awards outstanding under our Long-Term Incentive Plan may not tender the Shares underlying such awards in the Offer, unless and until the applicable award is vested, exercised or settled in Shares (as applicable), and the holder thereof has acquired the underlying Shares free of restrictions on the transfer of such Shares.

May I Tender Only A Portion Of The Shares That I Hold?

        Yes. You do not have to tender all of the Shares that you own to participate in the Offer.

Can I Tender My Shares At Different Prices?

        Yes. You can elect to tender some of your Shares at one price and an additional number of Shares at a second price. However, you cannot tender the same Shares at different prices. If you tender some Shares at one price and the other Shares at another price, you must use a separate Letter of Transmittal for each tender. See "Section 2. Procedure for Tendering Shares".

Until What Time Can I Withdraw Previously Tendered Shares?

        You can withdraw previously tendered Shares at any time until the Offer has expired, or until such later time and date to which the Offer is extended. In addition, unless we have already accepted your tendered Shares for payment, you may withdraw your tendered Shares at any time after 12:00 midnight, New York City time, at the end of the day on September 14, 2017. See "Section 3. Withdrawal Rights".

        You may give instructions to withdraw tendered Shares held in the OneBeacon 401(k) Plan at any time before 5:00 p.m. New York City time, on September 12, 2017, unless we extend the Offer, in which case you can give instructions to withdraw your Shares until 5:00 p.m., New York City time, on the date that is two business days before the new Expiration Time. See "Section 3. Withdrawal Rights".

How Do I Withdraw Previously Tendered Shares?

        To properly withdraw Shares, you must deliver a written notice of withdrawal with the required information to the Depositary while you still have the right to withdraw the Shares. If you have used more than one Letter of Transmittal or have otherwise tendered Shares in more than one group of Shares, you may withdraw Shares using either separate notices of withdrawal or a combined notice of withdrawal, so long as the required information is included. Some additional requirements apply if your Shares have been tendered under the procedure for book-entry transfer set forth in "Section 2. Procedures for Tendering Shares". If you have tendered your Shares by giving instructions to a bank, broker, dealer, trust company or other nominee, you must instruct such nominee to arrange for the withdrawal of your Shares. See "Section 3. Withdrawal Rights".

        If you wish to withdraw tendered Shares held in the OneBeacon 401(k) Plan, you must deliver an instruction form with the required information to the agent for the trustee of the OneBeacon 401(k) Plan, in accordance with the instructions in the letter furnished to you as a participant in the OneBeacon 401(k) Plan. If you wish to withdraw tendered Shares held in the OneBeacon 401(k) Plan, you must withdraw all such tendered Shares. See "Section 3. Withdrawal Rights".

What Does The Board Think Of The Offer?

        The Board makes no recommendation as to whether you should tender or refrain from tendering any or all of your Shares, or hold them, or as to the price or prices at which you may choose to tender your Shares. Rather, they encourage you to make your own decision after consulting with your own advisor as to whether or not to tender Shares, and if so, how many to tender and at what price or prices. In doing so, you should read carefully the information set forth or incorporated by reference in this Offer to Purchase and the Letter of Transmittal.

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Will Directors, Executive Officers Or Affiliates Of The Company Tender Shares In The Offer?

        All of our directors and executive officers have advised us that they do not intend to tender Shares pursuant to the Offer. Accordingly, if we complete the Offer, the proportional holdings of our directors and executive officers will increase. The Trust under the Executive Survivor Income Plan maintained by our subsidiary, OneBeacon Services, LLC, may elect to tender a number of Shares representing up to an aggregate value of $4.0 million. There can be no assurances that such Trust will tender Shares in the Offer or the price or prices at which any such Shares may be tendered. However, our directors and executive officers may, in compliance with applicable law, sell their Shares in open market transactions at prices that may or may not be more favorable than the Purchase Price to be paid to our shareholders in the Offer. See "Section 11. Interests of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares".

What Will Happen If I Do Not Tender My Shares?

        Shareholders who choose not to tender will own a greater percentage of our outstanding Shares following the consummation of the Offer. See "Section 5. Purpose of the Offer; Certain Effects of the Offer".

What Does The Company Intend To Do With The Purchased Shares After The Offer Expires?

        We will cancel the Shares. See "Section 5. Purpose of the Offer; Certain Effects of the Offer".

Does The Company Intend To Repurchase Any Shares Other Than Pursuant To The Offer During Or After The Offer?

        Rule 13e-4 of the Exchange Act generally prohibits us and our affiliates from purchasing any Shares, other than pursuant to the Offer, during the Offer and for the period ending ten business days after the expiration or termination of the Offer. Following that time, we may make Share repurchases from time to time through open market purchases, privately negotiated transactions, tender offers, exchange offers or otherwise. The Company has 643,130 Shares remaining under its existing Share repurchase authorization (which will not be reduced by purchases in the Offer). Whether we make additional Share repurchases will depend on many factors, including the number of Shares, if any, that we purchase in the Offer, the number of Shares our Board authorizes to be repurchased, our business and financial performance (including our liquidity and alternative uses for our resources) and situation, the business and market conditions at the time, including the price of the Shares, and such other factors as we may consider relevant. Any of these Share repurchases may be on the same terms or on terms that are more or less favorable to the selling shareholders in those transactions than the terms of the Offer. We cannot assure you as to which, if any, of these alternatives, or combinations thereof, we might pursue. See "Section 5. Purpose of the Offer; Certain Effects of the Offer".

Following The Offer, Will The Company Continue As A Public Company?

        We do not intend or believe that our purchase of Shares through the Offer will cause our remaining Shares to be delisted from the NYSE or cause us to be eligible for deregistration under the Exchange Act. See "Section 8. Conditions to the Offer".

What Is The Market Value Of My Shares As Of A Recent Date?

        On August 16, 2017, the last trading day before the public announcement of the Offer, the last reported closing sale price of the Shares reported on the NYSE was $850.65 per Share. The lower end of the price range for the Offer is below this last reported price per Share. We advise you to obtain a recent quotation for the Shares in deciding whether to tender your Shares. See "Section 7. Price Range of Shares; Dividends".

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What Are The United States Federal Income Tax Consequences If I Tender My Shares?

        The receipt of cash pursuant to the Offer generally will be treated for United States Federal income tax purposes either (a) as a sale or exchange eligible for capital gain or loss treatment or (b) as a dividend. Non-U.S. shareholders generally will not be subject to U.S. Federal income taxation on the receipt of cash pursuant to the Offer. See "Section 2. Procedures For Tendering Shares" and "Section 6. Certain U.S. Federal Income Tax Consequences".

What Is The Accounting Treatment Of The Offer?

        The accounting for the purchase of Shares pursuant to the Offer will result in a reduction of our common shareholders' equity in an amount equal to the aggregate purchase price of the Shares we purchase (plus expenses incurred in connection with the Offer) and a reduction in cash and cash equivalents in a corresponding amount. See "Section 5. Purpose of the Offer; Certain Effects of the Offer".

Will I Have To Pay Brokerage Commissions If I Tender My Shares?

        Tendering shareholders who hold Shares registered in their own name and who tender their Shares directly to the Depositary will not be obligated to pay brokerage commissions, solicitation fees or, except as set forth in "Section 4. Acceptance for Payment and Payment of Shares", stock transfer taxes on the purchase of Shares by us in the Offer. Shareholders holding Shares in a brokerage account or otherwise through brokers, dealers, commercial banks, trust companies or other nominees are urged to consult their brokers, dealers, commercial banks, trust companies or such other nominees to determine whether transaction costs may apply if shareholders tender Shares through such nominees and not directly to the Depositary. See "Section 2. Procedures for Tendering Shares", "Section 6. Certain U.S. Federal Income Tax Consequences" and "Section 15. Fees and Expenses".

Will I Have To Pay Stock Transfer Tax If I Tender My Shares?

        We will pay all stock transfer taxes, if any, payable on the transfer to us of Shares purchased pursuant to the Offer. If, however, payment of the Purchase Price is to be made to, or (in the circumstances permitted by the Offer) unpurchased Shares are to be registered in the name of, any person other than the registered holder, or if tendered book-entry accounts are registered in the name of any person other than the person signing the Letter of Transmittal, the amount of all stock transfer taxes, if any (whether imposed on the registered holder or the other person), payable on account of the transfer to the person will be deducted from the Purchase Price unless satisfactory evidence of the payment of the stock transfer taxes, or exemption from payment of the stock transfer taxes, is submitted. See "Section 4. Acceptance for Payment and Payment of Shares".

Who Can I Talk To If I Have Questions About The Offer?

        You can call D.F. King, the Information Agent for the Offer, at (800) 893-5865 (toll free) or either of the Dealer Managers at their respective telephone numbers listed on the back cover of this Offer to Purchase. For further contact information, see the back cover of this Offer to Purchase.

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FORWARD-LOOKING STATEMENTS

        This Offer to Purchase may contain "forward-looking statements". All statements, other than statements of historical facts, included or referenced in this release which address activities, events or developments which the Company expects or anticipates will or may occur in the future are forward-looking statements. The words "will", "believe", "intend", "expect", "anticipate", "project", "estimate", "predict" and similar expressions are also intended to identify forward-looking statements. These forward-looking statements include, among others, statements with respect to the Company's: change in adjusted book value per share or return on equity; business strategy; financial and operating targets or plans; incurred loss and loss adjustment expenses and the adequacy of its loss and loss adjustment expense reserves and related reinsurance; projections of revenues, income (or loss), earnings (or loss) per share, dividends, market share or other financial forecasts; expansion and growth of our business and operations; and future capital expenditures. These statements are based on certain assumptions and analyses made by the Company in light of its experience and perception of historical trends, current conditions and expected future developments, as well as other factors believed to be appropriate in the circumstances. However, whether actual results and developments will conform with its expectations and predictions is subject to a number of risks and uncertainties that could cause actual results to differ materially from expectations, including: the risk that OneBeacon's proposed merger with Intact Financial Corporation (the "OneBeacon Transaction") may not be completed on the currently contemplated timeline or at all; the possibility that any or all of the various conditions to the consummation of the OneBeacon Transaction may not be satisfied or waived, including the failure to receive any required regulatory approvals from any applicable governmental entities (or any conditions, limitations or restrictions placed on such approvals); the occurrence of any event, change or other circumstance that could give rise to the termination of the merger agreement dated May 2, 2017, among OneBeacon, Intact Financial Corporation and the other parties thereto (the "Merger Agreement"), including in circumstances which would require OneBeacon to pay a termination fee or other expenses; risks related to diverting management's attention from the Company's or OneBeacon's ongoing business operations and other risks related to the announcement or pendency of the OneBeacon Transaction, including on the Company's or OneBeacon's ability to retain and hire key personnel, their ability to maintain relationships with its customers, policyholders, brokers, service providers and others with whom they do business and their operating results and business generally; the risk that shareholder litigation in connection with the transactions contemplated by the Merger Agreement may result in significant costs of defense, indemnification and liability; the risks that are described from time to time in White Mountains's filings with the Securities and Exchange Commission, including but not limited to White Mountains's Annual Report on Form 10-K for the fiscal year ended December 31, 2016 filed February 27, 2017; claims arising from catastrophic events, such as hurricanes, earthquakes, floods, fires, terrorist attacks or severe winter weather; the continued availability of capital and financing; general economic, market or business conditions; business opportunities (or lack thereof) that may be presented to it and pursued; competitive forces, including the conduct of other property and casualty insurers and reinsurers; changes in domestic or foreign laws or regulations, or their interpretation, applicable to the Company, its competitors or its customers; an economic downturn or other economic conditions adversely affecting its financial position; recorded loss reserves subsequently proving to have been inadequate; actions taken by ratings agencies from time to time, such as financial strength or credit ratings downgrades or placing ratings on negative watch; and other factors, most of which are beyond the Company's control. Consequently, all of the forward-looking statements made in this Offer to Purchase are qualified by these cautionary statements, and there can be no assurance that the actual results or developments anticipated by the Company will be realized or, even if substantially realized, that they will have the expected consequences to, or effects on, the Company or its business or operations. Except for our obligations under Rule 13e-4(c)(3) and Rule 13e-4(e)(3) of the Exchange Act to disclose any material changes in the information previously disclosed to shareholders or as otherwise required by law, the Company assumes no obligation to

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publicly update any such forward-looking statements, whether as a result of new information, future events or otherwise.

        You should read this Offer to Purchase and the documents that we reference in this Offer to Purchase and have filed as exhibits to the Tender Offer Statement on Schedule TO filed with the SEC, of which this Offer to Purchase is a part, completely and with the understanding that our actual future results may be materially different from what we expect. We qualify all of our forward-looking statements by each of these cautionary statements.

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INTRODUCTION

To the Holders of Common Shares of White Mountains Insurance Group, Ltd.:

        White Mountains Insurance Group, Ltd. hereby offers to purchase 500,000 of its outstanding common shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest. We will select the lowest price per Share (in increments of $1.00) (the "Purchase Price") that will allow us to purchase the number of Shares sought in the Offer or, if a lesser number of Shares is properly tendered, all Shares that are properly tendered and not properly withdrawn. All Shares acquired in the Offer will be acquired at the same price, regardless of whether the shareholder tendered at a lower price. Our Offer is being made upon the terms and subject to the conditions set forth in this Offer to Purchase and in the related Letter of Transmittal (which together, as they may be amended or supplemented from time to time constitute the Offer).

        Only Shares properly tendered at prices at or below the Purchase Price and not properly withdrawn will be purchased. However, because of the proration and "odd lot" priority provisions described in this Offer to Purchase, not all of the Shares tendered at or below the Purchase Price may be purchased if more than the number of Shares we seek are properly tendered and not properly withdrawn. We will return any Shares that are tendered at prices in excess of the Purchase Price and Shares that we do not purchase because of proration promptly following the Expiration Time. See "Section 2. Procedures for Tendering Shares".

        Shareholders must, among other items, complete the section of the Letter of Transmittal relating to the price at which they are tendering Shares in order to properly tender Shares.

        OUR BOARD OF DIRECTORS (THE "BOARD") HAS APPROVED THE OFFER. HOWEVER, NONE OF THE COMPANY, OUR BOARD, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT MAKES ANY RECOMMENDATION AS TO WHETHER ANY SHAREHOLDER SHOULD TENDER OR REFRAIN FROM TENDERING SHARES PURSUANT TO THE OFFER, OR AS TO THE PRICE OR PRICES AT WHICH SHAREHOLDERS MAY CHOOSE TO TENDER SHARES. EACH SHAREHOLDER MUST MAKE HIS OR HER OWN DECISION AFTER CONSULTING WITH HIS OR HER OWN ADVISORS WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO TENDER AND THE PRICE OR PRICES AT WHICH SUCH SHAREHOLDER WILL TENDER THEM. IN DOING SO, SHAREHOLDERS SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE AND IN THE RELATED LETTER OF TRANSMITTAL INCLUDING THE PURPOSES AND EFFECTS OF THE OFFER. SEE "SECTION 5. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER".

        ALL OF OUR DIRECTORS AND EXECUTIVE OFFICERS HAVE ADVISED US THAT THEY DO NOT INTEND TO TENDER ANY SHARES OWNED BY THEM IN THE OFFER. ACCORDINGLY, IF WE COMPLETE THE OFFER, THE PROPORTIONAL HOLDINGS OF OUR DIRECTORS AND EXECUTIVE OFFICERS WILL INCREASE. THE TRUST UNDER THE EXECUTIVE SURVIVOR INCOME PLAN MAINTAINED BY OUR SUBSIDIARY, ONEBEACON SERVICES, LLC, MAY ELECT TO TENDER A NUMBER OF SHARES REPRESENTING UP TO AN AGGREGATE VALUE OF $4.0 MILLION. THERE CAN BE NO ASSURANCES THAT SUCH TRUST WILL TENDER SHARES IN THE OFFER OR THE PRICE OR PRICES AT WHICH ANY SUCH SHARES MAY BE TENDERED. HOWEVER, OUR DIRECTORS AND EXECUTIVE OFFICERS MAY, IN COMPLIANCE WITH APPLICABLE LAW, SELL THEIR SHARES IN OPEN MARKET TRANSACTIONS AT PRICES THAT MAY OR MAY NOT BE MORE FAVORABLE THAN THE PURCHASE PRICE TO BE PAID TO OUR SHAREHOLDERS. SEE "SECTION 11. INTERESTS OF DIRECTORS AND EXECUTIVE OFFICERS; TRANSACTIONS AND ARRANGEMENTS CONCERNING THE SHARES".

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        THE OFFER IS NOT CONDITIONED UPON THE RECEIPT OF FINANCING OR ANY MINIMUM NUMBER OF SHARES BEING TENDERED. THE OFFER IS, HOWEVER, SUBJECT TO CERTAIN CONDITIONS. SEE "SECTION 8. CONDITIONS TO THE OFFER".

        AS OF AUGUST 16, 2017, THERE WERE 4,336,625 SHARES OUTSTANDING. ACCORDINGLY, THE 500,000 SHARES WE ARE SEEKING TO PURCHASE THROUGH THIS OFFER REPRESENT APPROXIMATELY 12% OF THE SHARES OUTSTANDING AS OF SUCH DATE.

        In the event that more than 500,000 Shares are tendered in the Offer, we may exercise our right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of our outstanding Shares without extending the Expiration Time. We also expressly reserve the right, in our sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law.

        If more than 500,000 Shares (or such greater number of Shares as we may elect to purchase, subject to applicable law) are properly tendered at or below the Purchase Price and not properly withdrawn, we will accept Shares for purchase in the following order of priority:

•

First, we will purchase all "odd lots" of less than 100 Shares at the Purchase Price from shareholders who properly tender all of their Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time; and



•

Second, after purchasing all the "odd lots" that were properly tendered at or below the Purchase Price, we will purchase Shares at the Purchase Price from all other holders who properly tender Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time, on a pro rata basis, with appropriate adjustments to avoid purchases of fractional Shares, until we have acquired the number of Shares that we have offered to purchase.

        Therefore, we may not purchase all of the Shares that you tender even if you tender them at or below the Purchase Price.

        The Purchase Price will be paid, net to the seller in cash, less any applicable withholding taxes and without interest, for all Shares purchased. Tendering shareholders who hold Shares registered in their own name and who tender their Shares directly to the Depositary will not be obligated to pay brokerage commissions, solicitation fees or, except as set forth in "Section 4. Acceptance for Payment and Payment of Shares", stock transfer taxes on the purchase of Shares by us in the Offer. Shareholders holding Shares in a brokerage account or otherwise through brokers, dealers, commercial banks, trust companies or other nominees are urged to consult their brokers, dealers, commercial banks, trust companies or such other nominees to determine whether transaction costs may apply if shareholders tender Shares through such nominees and not directly to the Depositary. See "Section 2. Procedures for Tendering Shares", "Section 6. Certain U.S. Federal Income Tax Consequences" and "Section 15. Fees and Expenses".

        The Company will pay all reasonable charges and expenses of the Dealer Managers, the Information Agent and the Depositary incurred in connection with the Offer. All Shares tendered but not purchased in the Offer, including Shares tendered at or below the Purchase Price and Shares not purchased because of proration, will be returned to the tendering shareholders at the Company's expense promptly after the Expiration Time.

        The Shares are listed and traded on the NYSE under the symbol "WTM" and the Bermuda Stock Exchange under the symbol "WTM-BH". On August 16, 2017, the last full trading day before the public announcement of the Offer, the last reported closing sale price of the Shares on the NYSE was $850.65 per Share. Shareholders are urged to obtain a current market quotation for the Shares. See "Section 7. Price Range of Shares; Dividends".

        YOU SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE OR IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING OUR REASONS FOR MAKING THE OFFER. SEE "SECTION 5. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER".

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THE OFFER

        1. Number of Shares; Proration.    Upon the terms and subject to the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of such extension or amendment), the Company will accept for payment, and will pay for 500,000 Shares, or such lesser number of Shares as are properly tendered and not properly withdrawn in accordance with "Section 3. Withdrawal Rights", before the Expiration Time at a Purchase Price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest.

        The term "Expiration Time" means 12:00 midnight, New York City time, at the end of the day on September 14, 2017, unless and until the Company shall have extended the period during which the Offer is open, in which event the term "Expiration Time" shall mean the latest time and date at which the Offer, as so extended by the Company under Section 14, shall expire.

        If the Offer is oversubscribed as described below, Shares properly tendered at or below the Purchase Price and not properly withdrawn will be subject to proration, except for "odd lots". The proration period and, except as described herein, withdrawal rights, expire at the Expiration Time.

        Subject to all applicable laws and the terms and conditions of the Offer, the Company reserves the right, as described in "Section 8. Conditions to the Offer" and in "Section 14. Extension of the Offer; Termination; Amendment", to waive conditions thereto, in its sole discretion, and/or extend, withdraw, terminate, delay or amend the Offer in any case by making a public announcement thereof. There can be no assurance, however, that the Company will exercise its right to extend the Offer.

        In accordance with Instruction 7 to the Letter of Transmittal, shareholders desiring to tender Shares must either (i) specify that they are willing to sell their Shares to us at the price determined pursuant to the Offer or (ii) specify the price, not greater than $875 or less than $825 per Share (in increments of $1.00), at which they are willing to sell their Shares to us in the Offer. Promptly following the Expiration Time, we will, upon the terms and subject to the conditions of the Offer, determine a single per Share Purchase Price that we will pay for Shares properly tendered and not properly withdrawn pursuant to the Offer, taking into account the number of Shares tendered and the prices at which they are tendered. We will select the lowest purchase price specified by tendering shareholders that will allow us to purchase the number of Shares sought in the Offer or, if a lesser number of Shares is properly tendered, all Shares that are properly tendered and not properly withdrawn. All Shares purchased pursuant to the Offer will be purchased at the same Purchase Price regardless of whether the shareholder tendered at a lower price. If tendering shareholders wish to maximize the chance that their Shares will be purchased, they should check the box in the section of the Letter of Transmittal captioned "Shares Tendered at a Price Determined Pursuant to the Offer". Note that this election could result in the tendered Shares being purchased at the minimum price of $825 per Share.

        In the event that more than 500,000 Shares are properly tendered in the Offer, we may exercise our right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of our outstanding Shares without extending the Expiration Time. If we exercise our right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of our outstanding Shares, then we could have as few as 3,749,892 Shares outstanding following the purchase of the Shares tendered in the Offer. We also expressly reserve the right, in our sole discretion, to amend the Offer to purchase additional Shares, subject to applicable legal requirements.

        Only Shares properly tendered at prices at or below the Purchase Price and not properly withdrawn will be purchased. However, because of the proration provisions of the Offer, not all of the Shares tendered at or below the Purchase Price may be purchased if more than the number of Shares we seek are properly tendered and not properly withdrawn. All Shares tendered and not purchased in the Offer, including Shares tendered at or below the Purchase Price and Shares not purchased because

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of proration, will be returned to the tendering shareholders at our expense promptly following the Expiration Time.

        THE OFFER IS NOT CONDITIONED UPON THE RECEIPT OF FINANCING OR ANY MINIMUM NUMBER OF SHARES BEING TENDERED. THE OFFER IS, HOWEVER, SUBJECT TO CERTAIN OTHER CONDITIONS. SEE "SECTION 8. CONDITIONS TO THE OFFER".

        Priority of Purchases.    If more than 500,000 Shares (or such greater number of Shares as the Company may elect to purchase, subject to applicable law) are properly tendered at or below the Purchase Price and not properly withdrawn, we will accept Shares for purchase in the following order of priority:

•

First, we will purchase all "odd lots" of less than 100 Shares at the Purchase Price from shareholders who properly tender all of their Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time; and



•

Second, after purchasing all the "odd lots" that were properly tendered at or below the Purchase Price, we will purchase Shares at the Purchase Price from all other holders who properly tender Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time, on a pro rata basis, with appropriate adjustments to avoid purchases of fractional Shares, until we have acquired the number of Shares that we have offered to purchase.

        Therefore, we may not purchase all of the Shares that you tender even if you tender them at or below the Purchase Price.

        Odd Lots.    The term "odd lots" means all Shares tendered by any person (an "Odd Lot Holder") who owned beneficially or of record a total of fewer than 100 Shares and so certified in the appropriate place on the Letter of Transmittal and, if applicable, on the Notice of Guaranteed Delivery. To qualify for the odd lot preference, an odd lot holder must tender all Shares owned in accordance with the procedures described in Section 4. Odd lots will be accepted for payment before any proration of the purchase of other tendered Shares. Any Odd Lot Holder wishing to tender all of the shareholder's Shares in the Offer must complete the section entitled "Odd Lots" in the Letter of Transmittal and, if applicable, in the Notice of Guaranteed Delivery. See "Section 2. Procedures for Tendering Shares".

        Proration.    In the event that proration of tendered Shares is required, the Company will determine the final proration factor promptly after the Expiration Time. Proration for each shareholder tendering Shares (excluding Odd Lot Holders) will be based on the ratio of the number of Shares properly tendered and not properly withdrawn by the shareholder to the total number of Shares at or below the Purchase Price properly tendered and not properly withdrawn by all shareholders excluding Odd Lot Holders. Although the Company does not expect that it will be able to announce the final proration factor until a reasonable time after expiration of the period to complete tenders made by Guaranteed Delivery, it will announce preliminary results of proration by press release promptly after the Expiration Time. Shareholders may obtain such preliminary information from the Information Agent and may be able to obtain such information from their brokers or financial advisors.

        All Shares tendered but not purchased in to the Offer, including Shares tendered at or below the Purchase Price and Shares not purchased because of proration, will be returned to the tendering shareholders at the Company's expense promptly (which, in the event of proration, will not be until a reasonable period after the final proration factor has been calculated) following the Expiration Time.

        As described in "Section 6. Certain U.S. Federal Income Tax Consequences", the number of Shares that we will purchase from a shareholder pursuant to the Offer may affect the United States federal income tax consequences to the shareholder of the purchase and, therefore, may be relevant to a shareholder's decision whether or not to tender Shares. The Letter of Transmittal affords each

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shareholder who tenders Shares registered in such shareholder's name directly to the Depositary the opportunity to designate the order of priority in which Shares tendered are to be purchased in the event of proration. In the event the shareholder does not designate the order and fewer than all Shares are purchased due to proration, the Depositary will select the order of Shares purchased.

        2. Procedures for Tendering Shares.    General. Except as set forth below, in order for Shares to be properly tendered pursuant to the Offer, the Letter of Transmittal, properly completed and duly executed, together with any required signature guarantees (or, in the case of a book-entry transfer, an Agent's Message in lieu of the Letter of Transmittal) and any other documents required by the Letter of Transmittal, must be received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase and either (i) the Certificates evidencing tendered Shares must be received by the Depositary at such address or such Shares must be tendered pursuant to the procedure for book-entry transfer described below and a Book-Entry Confirmation must be received by the Depositary (including an Agent's Message if the tendering shareholder has not delivered a Letter of Transmittal), in each case on or prior to the Expiration Time, or (ii) the tendering shareholder must comply with the guaranteed delivery procedures described below. No alternative, conditional or contingent tenders will be accepted. The term "Agent's Message" means a message, transmitted by electronic means to, and received by, the Depositary and forming a part of a Book-Entry Confirmation that states that (i) DTC has received an express acknowledgment from the participant in DTC tendering the Shares that are the subject of such Book-Entry Confirmation, (ii) such participant has received and agrees to be bound by the terms of the Letter of Transmittal and (iii) the Company may enforce such agreement against such participant.

        In accordance with Instruction 7 to the Letter of Transmittal, each shareholder desiring to tender Shares in the Offer must either (i) check the box in the section of the Letter of Transmittal captioned "Shares Tendered at a Price Determined Pursuant to the Offer," in which case you will be deemed to have tendered your Shares at the minimum price of $825 per Share or (ii) check one of the boxes in the section of the Letter of Transmittal captioned "Shares Tendered at a Price Determined by Shareholder," indicating the price at which Shares are being tendered. A tender of Shares will be proper if, and only if, one of these boxes is checked on the Letter of Transmittal.

        If tendering shareholders wish to maximize the chance that their Shares will be purchased, they should check the box in the section on the Letter of Transmittal captioned "Shares Tendered At Price Determined Pursuant To The Offer". If you agree to accept the Purchase Price determined pursuant to the Offer, your Shares will be deemed to be tendered at the minimum price of $825 per share. You should understand that checking the box in the section of the Letter of Transmittal captioned "Shares Tendered at a Price Determined Pursuant to the Offer" may lower the Purchase Price paid for all purchased Shares in the Offer and could result in the tendered Shares being purchased at the minimum price of $825 per share, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.

        If tendering shareholders wish to indicate a specific price (in increments of $1.00) at which their Shares are being tendered, they must check a box under the section captioned "Price (in Dollars) per Share at Which Shares are Being Tendered". Tendering shareholders should be aware that this election could mean that none of their Shares will be purchased if they check a box other than the box representing the lowest price. A shareholder who wishes to tender Shares at more than one price must complete separate Letters of Transmittal for each price at which Shares are being tendered. The same Shares cannot be tendered (unless previously properly withdrawn in accordance with the terms of the Offer) at more than one price. In the event a holder who has submitted multiple Letters of Transmittal wishes to withdraw previously tendered shares in accordance with the terms of the Offer, separate notices of withdrawal (described in Section 3) are not required for each Letter of Transmittal unless each Letter of Transmittal tenders Shares at different prices; however, absent a notice of withdrawal, subsequent Letters of Transmittal do not revoke prior Letters of Transmittal. Shareholders may contact the Depositary for additional instructions.

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        Odd Lot Holders must tender all of their Shares and also complete the section captioned "Odd Lots" in the Letter of Transmittal and, if applicable, in the Notice of Guaranteed Delivery to qualify for the preferential treatment available to Odd Lot Holders as set forth in Section 1.

        Tendering shareholders who hold Shares registered in their own name and who tender their Shares directly to the Depositary will not be obligated to pay brokerage commissions, solicitation fees or, except as set forth in "Section 4. Acceptance for Payment and Payment of Shares", stock transfer taxes on the purchase of Shares by us in the Offer. Shareholders holding Shares in a brokerage account or otherwise through brokers, dealers, commercial banks, trust companies or other nominees are urged to consult their brokers, dealers, commercial banks, trust companies or such other nominees in order to tender such Shares and in order to determine whether transaction costs may apply if shareholders tender Shares through such nominees and not directly to the Depositary. It is likely that the nominee has established an earlier deadline for you to act to instruct the nominee to accept the Offer on your behalf.

        Procedure for the OneBeacon 401(k) Plan.    A participant in the OneBeacon 401(k) Plan may instruct the OneBeacon 401(k) Plan's trustee to tender some or all of the Shares allocated to such participant's account by completing and returning to the agent for such trustee an instruction form in accordance with the instructions in the letter furnished to participants the OneBeacon 401(k) Plan, in each case not later than two business days prior to the Expiration Time. All documents furnished to shareholders generally in connection with the Offer will be made available to participants whose OneBeacon 401(k) Plan accounts are credited with Shares. Although the Offer will remain open to all shareholders until the Expiration Time, if the trustee for the OneBeacon 401(k) Plan does not receive such participant's instructions by 5:00 p.m., New York City time, on September 12, 2017, such trustee will not tender Shares attributable to the participant's account unless the Offer is extended, in which case such participant's instruction must be received by 5:00 p.m., New York City time, on the date that is two business days before the new Expiration Time. Participants are urged to read the letter sent to them regarding and the separate instruction form carefully. Participants in the OneBeacon 401(k) Plan cannot use the Letter of Transmittal to direct the tender of Shares held under the OneBeacon 401(k) Plan, and must use the instruction form included in the separate letter sent to them. Participants in the OneBeacon 401(k) Plan who also hold Shares outside the OneBeacon 401(k) Plan must (i) complete an instruction form according to the instructions in the letter sent to them for Shares held under the OneBeacon 401(k) Plan and (ii) use the applicable procedures otherwise described in this Offer to Purchase to tender Shares outside the OneBeacon 401(k) Plan.

        Book-Entry Transfer.    The Depositary will establish accounts with respect to the Shares at DTC for purposes of the Offer within two business days after the date of this Offer to Purchase. Any financial institution that is a participant in DTC's system may make a book-entry delivery of Shares by causing DTC to transfer such Shares into the Depositary's account in accordance with DTC's procedures for such transfer. However, although delivery of Shares may be effected through book-entry transfer at DTC, either the Letter of Transmittal, properly completed and duly executed, together with any required signature guarantees, or an Agent's Message in lieu of the Letter of Transmittal, and any other required documents, must, in any case, be received by the Depositary at one of its addresses set forth on the back cover of this Offer to Purchase prior to the Expiration Time, or the tendering shareholder must comply with the guaranteed delivery procedure described below. DELIVERY OF DOCUMENTS TO DTC DOES NOT CONSTITUTE DELIVERY TO THE DEPOSITARY.

        Signature Guarantees.    Signatures on all Letters of Transmittal must be guaranteed by a firm which is a member of the Securities Transfer Agent Medallion Program, the New York Stock Exchange Medallion Signature Program or the Stock Exchange Medallion Program (each, an "Eligible Institution"), except in cases where Shares are tendered (i) by a registered holder (which term, for purposes of this Section 2, will include any participant in the DTC whose name appears on a security

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position listing as the owner of the Shares) of Shares who has not completed either the box entitled "Special Payment Instructions" or the box entitled "Special Delivery Instructions" on the Letter of Transmittal or (ii) for the account of an Eligible Institution.

        If a Certificate is registered in the name of a person other than the signatory of the Letter of Transmittal, or if payment is to be made, or a Certificate not accepted for payment or not tendered is to be returned, to a person other than the registered holder(s), then the Certificate must be endorsed or accompanied by appropriate share powers, in either case, signed exactly as the name(s) of the registered holder(s) appear on the Certificate, with the signature(s) on such Certificate or share powers guaranteed by an Eligible Institution. If the Letter of Transmittal or share powers are signed or any Certificate is endorsed by trustees, executors, administrators, guardians, attorneys-in-fact, agents, officers of corporations or others acting in a fiduciary or representative capacity, such persons should so indicate when signing and, unless waived by the Company, proper evidence satisfactory to the Company of their authority to so act must be submitted. See Instructions 1 and 5 of the Letter of Transmittal.

        Guaranteed Delivery.    If a shareholder desires to tender Shares pursuant to the Offer and the Certificates evidencing such shareholder's Shares are not immediately available or such shareholder cannot deliver the Certificates and all other required documents to the Depositary prior to the Expiration Time, or such shareholder cannot complete the procedure for delivery by book-entry transfer on a timely basis, such Shares may nevertheless be tendered, provided that all the following conditions are satisfied:

(i)

such tender is made by or through an Eligible Institution;



(ii)

a properly completed and duly executed Notice of Guaranteed Delivery, substantially in the form provided by the Company, is received prior to the Expiration Time by the Depositary as provided below; and



(iii)

the Certificates (or a Book-Entry Confirmation) evidencing all tendered Shares, in proper form for transfer, in each case together with the Letter of Transmittal, properly completed and duly executed, with any required signature guarantees (or, in connection with a book-entry transfer, an Agent's Message), and any other documents required by the Letter of Transmittal are received by the Depositary within two business days after the date of execution of such Notice of Guaranteed Delivery.

        The Notice of Guaranteed Delivery may be delivered by mail, express or overnight delivery, or transmitted by email to the Depositary and must include a guarantee by an Eligible Institution in the form set forth in the form of Notice of Guaranteed Delivery made available by the Company.

        In all cases, payment for Shares tendered and accepted for payment pursuant to the Offer will be made only after timely receipt by the Depositary of the Certificates evidencing such Shares, or a Book-Entry Confirmation of the delivery of such Shares, and the Letter of Transmittal, properly completed and duly executed, with any required signature guarantees, and any other documents required by the Letter of Transmittal.

        Shareholders may contact the Information Agent or their broker for assistance. The contact information for the Information Agent is on the back cover page of this Offer to Purchase.

        THE METHOD OF DELIVERY OF CERTIFICATES AND ALL OTHER REQUIRED DOCUMENTS, INCLUDING DELIVERY THROUGH DTC, IS AT THE OPTION AND RISK OF THE TENDERING SHAREHOLDER, AND THE DELIVERY WILL BE DEEMED MADE ONLY WHEN ACTUALLY RECEIVED BY THE DEPOSITARY (INCLUDING, IN THE CASE OF BOOK-ENTRY TRANSFER, BY BOOK-ENTRY CONFIRMATION). IF DELIVERY IS BY MAIL, REGISTERED MAIL WITH RETURN RECEIPT REQUESTED, PROPERLY INSURED, IS RECOMMENDED. IN

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ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY DELIVERY TO THE DEPOSITARY PRIOR TO THE EXPIRATION TIME.

        Determination of Validity.    All questions as to the price to be paid for the Shares to be accepted and the validity, form, eligibility (including time of receipt) and acceptance for payment of any tender of Shares will be determined by the Company in its reasonable discretion, which determination shall be final and binding on all parties. The Company reserves the absolute right to reject any and all tenders reasonably determined by it not to be in proper form or the acceptance for payment of which may, in the opinion of its counsel, be unlawful. The Company also reserves the absolute right to waive any condition of the Offer (except as provided in "Section 8. Conditions to the Offer") or any defect or irregularity in the tender of any particular Shares or any particular shareholder, whether or not similar defects or irregularities are waived in the case of other shareholders, and the Company's reasonable interpretation of the terms and conditions of the Offer will be final and binding on all persons. No tender of Shares will be deemed to have been properly made until all defects and irregularities have been cured or waived to the satisfaction of the Company. We will not be liable for failure to waive any condition of the Offer, or any defect or irregularity in any tender of Shares. Neither the Company, nor any other person will be under any duty to give notification of any defects or irregularities in tenders or incur any liability for failure to give any such notification. Our reasonable interpretation of the terms of and conditions to the Offer, including the Letter of Transmittal and the instructions thereto, will be final and binding on all persons participating in the Offer. Any determination by the Company as to the validity, form, eligibility and acceptance of Shares for payment, or any interpretation by the Company as to the terms and conditions of the Offer, is subject to applicable law and, if challenged by shareholders in a lawsuit, to the judgment of a court of competent jurisdiction.

        Tendering Shareholder's Representation and Warranty; Acceptance by the Company Constitutes an Agreement.    A tender of Shares pursuant to any of the procedures described above will constitute the tendering shareholder's acceptance of the terms and conditions of the Offer, as well as the tendering shareholder's representation and warranty to the Company that (i) the shareholder has a "net long position," within the meaning of Rule 14e-4 promulgated by the SEC under the Exchange Act, in the Shares or equivalent securities at least equal to the Shares being tendered and (ii) the tender of Shares complies with Rule 14e-4. It is a violation of Rule 14e-4 for a person, directly or indirectly, to tender Shares for that person's own account unless, at the time of tender and at the end of the proration period or period during which Shares are accepted by lot (including any extensions thereof), the person so tendering (i) has a net long position equal to or greater than the amount of (a) Shares tendered or (b) other securities convertible into or exchangeable or exercisable for the Shares tendered and will acquire the Shares for tender by conversion, exchange or exercise and (ii) will deliver or cause to be delivered the Shares in accordance with the terms of the Offer. Rule 14e-4 provides a similar restriction applicable to the tender or guarantee of a tender on behalf of another person. Our acceptance for payment of Shares tendered pursuant to the Offer will constitute a binding agreement between the tendering shareholder and us upon the terms and conditions of the Offer.

        Backup Federal Income Tax Withholding.    Under the United States Federal income tax backup withholding rules, 28% of the gross proceeds payable to a shareholder or other payee pursuant to the Offer must be withheld and remitted to the United States Treasury, unless the shareholder or other payee provides his or her taxpayer identification number (employer identification number or social security number) to the Depositary and certifies that such number is correct or an exemption otherwise applies under applicable regulations. Therefore, unless an exemption exists and is proven in a manner satisfactory to the Depositary, each tendering shareholder should complete and sign the Substitute Form W-9 included as part of the Letter of Transmittal so as to provide the information and certification necessary to avoid backup withholding. Certain shareholders (including, among others, all corporations and certain foreign individuals) are not subject to these backup withholding and reporting requirements. In order for a foreign individual to qualify as an exempt recipient, that shareholder must

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submit a statement (generally, an IRS Form W-8BEN or W-8BEN-E), signed under penalties of perjury, attesting to that individual's exempt status. Such statements can be obtained from the Depositary. See "Important Tax Information" and "Substitute Form W-9" in the Letter of Transmittal.

        ANY TENDERING SHAREHOLDER OR OTHER PAYEE WHO FAILS TO COMPLETE FULLY AND SIGN THE SUBSTITUTE FORM W-9 INCLUDED IN THE LETTER OF TRANSMITTAL MAY BE SUBJECT TO REQUIRED UNITED STATES FEDERAL INCOME TAX BACKUP WITHHOLDING OF 28% OF THE GROSS PROCEEDS PAID TO SUCH SHAREHOLDER OR OTHER PAYEE PURSUANT TO THE TENDER OFFER.

        Other Requirements.    If the Certificates which a registered holder wants to surrender have been lost, stolen, destroyed or mutilated, the shareholder should follow the instructions set forth in the Letter of Transmittal.

        CERTIFICATES FOR SHARES, TOGETHER WITH A PROPERLY COMPLETED AND DULY EXECUTED LETTER OF TRANSMITTAL, WITH ANY REQUIRED SIGNATURE GUARANTEES, OR AN AGENT'S MESSAGE, AND ANY OTHER DOCUMENTS REQUIRED BY THE LETTER OF TRANSMITTAL, MUST BE DELIVERED TO THE DEPOSITARY AND NOT TO US OR THE INFORMATION AGENT. ANY SUCH DOCUMENTS DELIVERED TO US OR THE INFORMATION AGENT WILL NOT BE FORWARDED TO THE DEPOSITARY AND THEREFORE WILL NOT BE DEEMED TO BE PROPERLY TENDERED.

        3. Withdrawal Rights.    Tenders of the Shares made pursuant to the Offer are irrevocable, except that such Shares may be withdrawn at any time prior to the Expiration Time and, unless theretofore accepted for payment by the Company pursuant to the Offer, may also be withdrawn at any time after 12:00 midnight, New York City time, at the end of the day on September 14, 2017. If the Company extends the Offer, is delayed in its acceptance for payment of Shares or is unable to accept Shares for payment pursuant to the Offer for any reason, then, without prejudice to the Company's rights under the Offer, the Depositary may, nevertheless, on behalf of the Company, retain tendered Shares, and such Shares may not be withdrawn except to the extent that tendering shareholders are entitled to withdrawal rights as described in this Section 3.

        For a withdrawal to be effective, a written notice of withdrawal must be timely received by the Depositary at one of its addresses set forth on the back cover page of this Offer to Purchase. Any such notice of withdrawal must specify the name of the person who tendered the Shares to be withdrawn, the number of Shares to be withdrawn and, if different from that of the person who tendered such Shares, the name of the registered holder of such Shares. A shareholder who has tendered Shares at more than one price must complete a separate notice of withdrawal for Shares tendered at each price. If Certificates evidencing Shares to be withdrawn have been delivered or otherwise identified to the Depositary, then, prior to the physical release of such Certificates, the serial numbers shown on such Certificates must be submitted to the Depositary and the signature(s) on the notice of withdrawal must be guaranteed by an Eligible Institution, unless such Shares have been tendered for the account of an Eligible Institution. If Shares have been tendered pursuant to the procedure for book-entry transfer as set forth in "Section 2. Procedures for Tendering Shares", any notice of withdrawal must specify the name and number of the account at DTC to be credited with the withdrawn Shares or must otherwise comply with DTC's procedures.

        Withdrawals of tenders of Shares may not be rescinded, and Shares properly withdrawn will thereafter be deemed not properly tendered for purposes of the Offer. However, withdrawn Shares may be retendered by following the procedures described in "Section 2. Procedures for Tendering Shares", at any time prior to the Expiration Time.

        If you wish to withdraw tendered Shares held in the OneBeacon 401(k) Plan, you must deliver an instruction form with the required information to the agent for the trustee of the OneBeacon 401(k)

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Plan by 5:00 p.m., New York City time, on September 12, 2017, unless we extend the Offer, in which case you can instruct the trustee to withdraw your Shares until 5:00 p.m., New York City time, on the date that is two business days before the new Expiration Time. If you wish to withdraw tendered Shares held in the OneBeacon 401(k) Plan, you must withdraw all such tendered Shares. If you withdraw Shares held in the OneBeacon 401(k) Plan, you may retender such Shares by giving instruction to the trustee within the applicable time period as described in "Section 2. Procedures for Tendering Shares".

        All questions as to the form and validity (including time of receipt) of any notice of withdrawal will be determined by the Company in its reasonable discretion, which determination will be final and binding on all parties. None of the Company, the Depositary, the Information Agent or any other person will be under any duty to give notification of any defects or irregularities in any notice of withdrawal or incur any liability for failure to give any such notification. Any determination by the Company as to the form and validity (including time of receipt) of any notice of withdrawal, or any interpretation by the Company as to the terms and conditions of the Offer, is subject to applicable law and, if challenged by shareholders in a lawsuit, to the judgment of a court of competent jurisdiction.

        4. Acceptance for Payment and Payment for Shares.    Upon the terms and subject to the conditions of the Offer, promptly following the Expiration Time, we (i) will determine which shareholders tendered Shares at or below the Purchase Price and (ii) will accept for payment and pay for (and thereby purchase) up to 500,000 Shares (or such greater number of Shares as we may elect to purchase, subject to applicable law) which are properly tendered at prices at or below the Purchase Price and not properly withdrawn on or before the Expiration Time.

        In all cases, payment for Shares tendered and accepted for payment pursuant to the Offer will be made promptly, subject in the event of proration to the time necessary to determine the applicable proration factor, but only after timely receipt by the Depositary of (i) the Certificates or timely confirmation (a "Book-Entry Confirmation") of a book-entry transfer of such Shares into the Depositary's account at DTC pursuant to the procedures set forth in "Section 2. Procedures for Tendering Shares", (ii) the Letter of Transmittal, properly completed and duly executed, with any required signature guarantees or, in the case of a book-entry transfer, an Agent's Message (as defined below) in lieu of the Letter of Transmittal and (iii) any other documents required by the Letter of Transmittal.

        In the event of proration, we will determine the proration factor and pay for those tendered Shares accepted for payment promptly after the Expiration Time. However, we do not expect to be able to announce the final results of any proration and commence payment for Shares purchased until a reasonable time after expiration of the period to complete tenders made by Guaranteed Delivery.

        For purposes of the Offer, the Company will be deemed to have accepted for payment, and thereby purchased, subject to the Odd Lot priority and proration, Shares properly tendered and not properly withdrawn, if and when the Company gives oral or written notice to the Depositary, as agent for the tendering shareholders, of the Company's acceptance for payment of such Shares pursuant to the Offer. Upon the terms and subject to the conditions of the Offer, payment for Shares accepted for payment pursuant to the Offer will be made by deposit of the Purchase Price with the Depositary, which will act as agent for tendering shareholders for the purpose of receiving payments from the Company and transmitting such payments to tendering shareholders whose Shares have been accepted for payment.

        Under no circumstances will we pay interest on the Purchase Price, including but not limited to, by reason of any delay in making payment.

        We will pay all stock transfer taxes, if any, payable on the transfer to us of Shares purchased pursuant to the Offer. If, however, payment of the Purchase Price is to be made to, or (in the circumstances permitted by the Offer) unpurchased Shares are to be registered in the name of, any

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person other than the registered holder, or if tendered book-entry accounts are registered in the name of any person other than the person signing the Letter of Transmittal, the amount of all stock transfer taxes, if any (whether imposed on the registered holder or the other person), payable on account of the transfer to the person will be deducted from the Purchase Price unless satisfactory evidence of the payment of the stock transfer taxes, or exemption from payment of the stock transfer taxes, is submitted.

        If any tendered Shares are not accepted for payment for any reason pursuant to the terms and conditions of the Offer (including those not purchased because of proration or because Shares are tendered at prices in excess of the Purchase Price), or if Certificates are submitted evidencing more Shares than are tendered, unless a shareholder specified otherwise in the Letter of Transmittal, Certificates evidencing unpurchased Shares will be returned, without expense to the tendering shareholder (or, in the case of Shares tendered by book-entry transfer into the Depositary's account at DTC pursuant to the procedure set forth in "Section 2. Procedures for Tendering Shares", such Shares will be credited to an account maintained at DTC) within a reasonable time after determination of the final proration factor.

        5. Purpose of the Offer; Certain Effects of the Offer.    The Offer is being made to utilize a portion of the Company's undeployed capital and to provide shareholders with added liquidity. The Company recognizes that its Shares are not widely held and are not regularly followed by many analysts. Based on existing circumstances, the Company believes that it currently has adequate capital and liquidity to fund the maximum amount contemplated by the Offer and to meet its ongoing needs. Accordingly, we have determined that it is in the interest of the Company's shareholders to create a selling opportunity for shareholders through a repurchase by the Company of up to 500,000 Shares. The Offer represents the opportunity for us to return capital to shareholders who elect to tender their Shares. The Offer will also afford to shareholders the opportunity to dispose of Shares without the usual transaction costs associated with any market sale; however, if you hold your Shares through a broker, dealer, commercial bank, trust company or other nominee and your nominee tenders Shares on your behalf, your nominee may charge you a fee for doing so.

        The Offer will further allow qualifying Odd Lot Holders whose Shares are purchased pursuant to the Offer to avoid the payment of brokerage commissions and any applicable odd-lot discount payable on a sale of Shares in a transaction effected on a securities exchange.

        To the extent the purchase of Shares in the Offer results in a reduction in the number of shareholders of record, the costs to the Company for services to shareholders will be reduced. We can give no assurance, however, that we will not issue additional Shares or equity interests in the future. Shareholders may be able to sell non-tendered Shares in the future on the NYSE or otherwise, at a net price which may be higher than the Purchase Price in the Offer. We can give no assurance, however, as to the price at which a shareholder may be able to sell his or her Shares in the future, which may be higher or lower than the Purchase Price paid by us in the Offer.

        The accounting for the purchase of Shares pursuant to the Offer will result in a reduction of our common shareholders' equity in an amount equal to the aggregate purchase price of the Shares we purchase (plus expenses incurred in connection with the Offer) and a reduction in cash and cash equivalents in a corresponding amount.

        Shareholders whose Shares are not purchased in the Offer will obtain an increase in their ownership interest in the Company and thus in the Company's future earnings and assets because Shares purchased pursuant to the Offer will be cancelled.

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        OUR BOARD HAS APPROVED THE OFFER. HOWEVER, NONE OF THE COMPANY, OUR BOARD, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT MAKES ANY RECOMMENDATION AS TO WHETHER ANY SHAREHOLDER SHOULD TENDER OR REFRAIN FROM TENDERING SHARES PURSUANT TO THE OFFER, OR AS TO THE PURCHASE PRICE OR PRICES AT WHICH SHAREHOLDERS MAY CHOOSE TO TENDER SHARES. EACH SHAREHOLDER MUST MAKE HIS OR HER OWN DECISION AFTER CONSULTING WITH HIS OR HER OWN ADVISORS WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO TENDER AND THE PRICE OR PRICES AT WHICH SUCH SHAREHOLDER WILL TENDER THEM. IN DOING SO, SHAREHOLDERS SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE AND IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING THE PURPOSES AND EFFECTS OF THE OFFER. SEE "SECTION 5. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER".

        ALL OF OUR DIRECTORS AND EXECUTIVE OFFICERS HAVE ADVISED US THAT THEY DO NOT INTEND TO TENDER ANY SHARES OWNED BY THEM IN THE OFFER. THE TRUST UNDER THE EXECUTIVE SURVIVOR INCOME PLAN MAINTAINED BY OUR SUBSIDIARY, ONEBEACON SERVICES, LLC, MAY ELECT TO TENDER A NUMBER OF SHARES REPRESENTING UP TO AN AGGREGATE VALUE OF $4.0 MILLION. THERE CAN BE NO ASSURANCES THAT SUCH TRUST WILL TENDER SHARES IN THE OFFER OR THE PRICE OR PRICES AT WHICH ANY SUCH SHARES MAY BE TENDERED. SEE "SECTION 11. INTERESTS OF DIRECTORS AND EXECUTIVE OFFICERS; TRANSACTIONS AND ARRANGEMENTS CONCERNING THE SHARES".

        Shares acquired by the Company pursuant to the Offer will be cancelled. The purchase of up to 500,000 Shares pursuant to the Offer will not cause the Shares to be delisted by the NYSE or deregistered under the Exchange Act. See "Section 8. Conditions to the Offer".

        Rule 13e-4 of the Exchange Act generally prohibits us and our affiliates from purchasing any Shares, other than pursuant to the Offer, during the Offer and for the period ending ten business days after the expiration or termination of the Offer. Following that time, we may make Share repurchases from time to time through open market purchases, privately negotiated transactions, tender offers, exchange offers or otherwise. The Company has 643,130 Shares remaining under its existing Share repurchase authorization (which will not be reduced by purchases in the Offer). Whether we make additional Share repurchases will depend on many factors, including the number of Shares, if any, that we purchase in the Offer, the number of Shares our Board authorizes to be repurchased, our business and financial performance (including our liquidity and alternative uses for our resources) and situation, the business and market conditions at the time, including the price of the Shares, and such other factors as we may consider relevant. Any of these Share repurchases may be on the same terms or on terms that are more or less favorable to the selling shareholders in those transactions than the terms of the Offer. We cannot assure you as to which, if any, of these alternatives, or combinations thereof, we might pursue.

        Other Plans.    On May 2, 2017, OneBeacon, a non-wholly owned subsidiary of the Company, announced it had entered into a definitive agreement to be acquired by Intact Financial Corporation, a Canadian Corporation ("Intact"), in an all-cash transaction for $18.10 per share, or roughly 1.65x tangible book value (the "OneBeacon Transaction"). The OneBeacon Transaction was approved by OneBeacon shareholders on June 18, 2017. White Mountains expects to receive gross proceeds of $1.3 billion from the OneBeacon Transaction, which is expected to close in the fourth quarter of 2017. It is subject to regulatory approval and other customary closing conditions. As we have previously disclosed, we currently intend to distribute approximately $1.5 to $2.0 billion to our shareholders over the medium-term. In addition, consistent with our fundamental philosophy, we remain focused on creating shareholder value by pursuing value-oriented, opportunistic transactions. In the ordinary course

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of our business, we review and assess transactions that we believe may provide value to our shareholders on a continual basis. Over the next several years, we currently intend to utilize approximately $200 million of our undeployed capital per year, or approximately $1.0 to $1.5 billion in the aggregate, to engage in transactions that we believe will provide value to our shareholders, which may include, among other things, monetizing all or part of our existing investments, entering into joint ventures in innovative areas, or purchasing majority stakes in operating businesses that we understand and can enhance, primarily in insurance, broader financial services, and adjacent sectors. There can be no assurance of when or whether any such transactions will be consummated by us. In addition, the size and type of transactions in which we engage may vary significantly and it is likely that we will utilize in excess of, or less than, $200 million of undeployed capital in any year and that we may utilize in excess of, or less than, $1.0 to $1.5 billion of undeployed capital in the aggregate over the next several years. Except with respect to the OneBeacon Transaction, or as otherwise disclosed in the Offer or in our filings with the SEC incorporated herein by reference (see "Section 9. Certain Information Concerning the Company"), we do not have any plans, proposals or negotiations under way that would relate to or result in: (i) any extraordinary transaction (such as a merger, reorganization or liquidation) involving us or any of our subsidiaries; (ii) any purchase, sale or transfer of a material amount of assets of the Company or any of our subsidiaries; (iii) any change in the Board or management or material term of the employment contract of any executive officer; (iv) any material change in our current dividend rate or policy or capitalization, indebtedness, corporate structure or business; (v) any class of our equity securities being delisted from the NYSE or the Bermuda Stock Exchange or ceasing to be authorized to be quoted on the NYSE or the Bermuda Stock Exchange; (vi) any class of our equity securities becoming eligible for termination of registration under Section 12(g) of the Securities Act of 1933; (vii) the suspension of our obligation to file reports under the Section 15(d) of the Exchange Act; (viii) the acquisition or disposition of any securities issued by the Company; or (ix) any changes in our articles of incorporation, bylaws or other governing instruments or other actions that could impede the acquisition of control of the Company.

        6. Certain U.S. Federal Income Tax Consequences.    This is a general summary of the material U.S. Federal income tax consequences of the sale of Shares pursuant to the Offer applicable to "U.S. Holders" as defined below. This discussion is based on the Internal Revenue Code of 1986, as amended (the "Code"), applicable Treasury Regulations and administrative and judicial interpretations, all as of the date hereof and all of which are subject to change (possibly on a retroactive basis). This summary does not discuss all the tax consequences that may be relevant to a particular shareholder in light of the shareholder's particular circumstances, including the impact of the Medicare contribution tax on net investment income. Different rules that are not discussed below may apply to some shareholders subject to special tax rules such as partnerships (or entities classified as partnerships for U.S. Federal income tax purposes), insurance companies, tax-exempt persons, financial institutions, regulated investment companies, dealers in securities or currencies, persons that hold Shares as a position in a "straddle" or as part of a "hedge", "conversion transaction" or other integrated investment, persons who received Shares as compensation or persons whose functional currency is other than the United States dollar. This summary does not address any state, local or foreign tax or alternative minimum tax considerations that may be relevant to a shareholder's decision to tender Shares pursuant to the Offer. This summary assumes Shares are held as capital assets within the meaning of Section 1221 of the Code.

        EACH SHAREHOLDER IS URGED TO CONSULT HIS OR HER OWN TAX ADVISOR WITH RESPECT TO THE U.S. FEDERAL, STATE AND LOCAL TAX CONSEQUENCES OF PARTICIPATING IN THE OFFER, AS WELL AS ANY TAX CONSEQUENCES ARISING UNDER THE LAWS OF ANY OTHER TAXING JURISDICTION.

        A "U.S. Holder" is a beneficial owner of Shares who is:

(i)

a citizen or individual resident of the United States;

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(ii)

a corporation (or other business entity treated as a corporation for U.S. Federal income tax purposes) created or organized in or under the laws of the United States, any state thereof, or the District of Columbia;



(iii)

an estate the income of which is subject to U.S. Federal income tax regardless of its source; or



(iv)

a trust that (A) is subject to the primary supervision of a court within the United States and the control of one or more U.S. persons or (B) otherwise has validly elected to be treated as a U.S. domestic trust for U.S. Federal income tax purposes.

        A "Non-U.S. Holder" is a beneficial owner of Shares who is not a U.S. Holder.

        The U.S. Federal income tax treatment of a partner in a partnership (or an entity or arrangement treated as a partnership for U.S. Federal income tax purposes) that holds Shares will depend on the status of the partner and the activities of the partnership. Prospective participants in the Offer that are partnerships (or entities or arrangements treated as partnerships for U.S. Federal income tax purposes) should consult their own tax advisors concerning the U.S. Federal income tax consequences to them and their partners of the participation in the Offer by the partnership.

        Tax Consequences of Offer—Distribution vs. Sale Treatment.    The Company's purchase of Shares from a U.S. Holder pursuant to the Offer will be treated either as a sale of the Shares or as a distribution by the Company. The purchase will be treated as a sale if the U.S. Holder meets any of the three tests discussed below. It will be treated as a distribution if the U.S. Holder satisfies none of those tests.

        If the purchase of Shares from a U.S. Holder is treated as a sale, the U.S. Holder will recognize gain or loss equal to the difference between the amount of cash received by the U.S. Holder and the U.S. Holder's tax basis in the Shares sold. The gain or loss will be capital gain or loss and will be long-term capital gain or loss if the Shares were held more than one year. A U.S. Holder must calculate gain or loss separately for each block of Shares that he or she owns, and the gains and losses for each block may not be netted in determining overall gain or loss. A U.S. Holder may be able to designate which blocks and the order of such blocks of Shares to be tendered pursuant to the Offer. The gain or loss will be U.S. source for foreign tax credit purposes.

        If the purchase of Shares from a U.S. Holder is treated as a distribution by the Company, the full amount of cash received by the U.S. Holder for the Shares (without any offset for the U.S. Holder's tax basis in the purchased Shares) will be treated as a dividend to the extent of the Company's current and accumulated earnings and profits allocable to the distribution. The tax basis of the U.S. Holder's sold Shares will be added to the tax basis of his or her remaining Shares. The Company believes that it has sufficient current and accumulated earnings and profits so that all purchases treated as distributions will be treated as dividends. For non-corporate U.S. Holders, such dividends may be "qualified dividend income" that is taxed at the lower applicable capital gains rate provided that certain conditions are satisfied, including certain holding period requirements. The Company believes that it is currently a "U.S.-owned foreign corporation" for U.S. Federal income tax purposes. As a result, a portion of any dividends may be treated as U.S. source for foreign tax credit purposes. To the extent, if any, payments made by the Company exceed a U.S. Holder's allocable share of the Company's current and accumulated earnings and profits, the distribution will first be treated as a non-taxable return of capital, causing a reduction in the U.S. Holder's adjusted basis in his or her Shares, and any amounts in excess of the U.S. Holder's adjusted basis will be treated as capital gain. Shareholders should consult their own tax advisors concerning the rules discussed in this paragraph in their particular circumstances.

        Determination of Sale or Distribution Treatment.    The Company's purchase of Shares pursuant to the Offer will be treated as a sale of the Shares by a U.S. Holder if:

(a)

the purchase completely terminates the U.S. Holder's equity interest in the Company;

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(b)

the receipt of cash by the U.S. Holder is "not essentially equivalent to a dividend"; or



(c)

as a result of the purchase there is a "substantially disproportionate" reduction in the U.S. Holder's equity interest in the Company.

        If none of these tests are met with respect to a particular U.S. Holder, then the Company's purchase of Shares from that U.S. Holder pursuant to the Offer will be treated as a distribution.

        In applying these tests, the constructive ownership rules of Section 318 of the Code apply. Thus, a U.S. Holder is treated as owning not only Shares actually owned by the U.S. Holder but also Shares actually (and in some cases constructively) owned by others. Under the constructive ownership rules, a U.S. Holder will be considered to own Shares owned, directly or indirectly, by certain members of the U.S. Holder's family and certain entities (such as corporations, partnerships, trusts and estates) in which the U.S. Holder has an equity interest, as well as Shares that the U.S. Holder has an option to purchase.

        It may be possible for a tendering U.S. Holder to satisfy one of these three tests by contemporaneously selling or otherwise disposing of all or some of the Shares that such U.S. Holder actually or constructively owns that are not purchased pursuant to the Offer. Correspondingly, a tendering U.S. Holder may not be able to satisfy one of these three tests because of contemporaneous acquisitions of Shares by such U.S. Holder or a related party whose Shares are attributed to such U.S. Holder. Shareholders should consult their own tax advisors regarding the tax consequences of such sales or acquisitions in their particular circumstances.

        Complete Termination.    A purchase of Shares pursuant to the Offer will result in a "complete termination" of the U.S. Holder's interest in the Company if, immediately after the sale, either:

(a)

the U.S. Holder owns, actually and constructively, no Shares; or



(b)

the U.S. Holder actually owns no Shares and effectively waives constructive ownership of any constructively owned Shares under the procedures described in Section 302(c)(2) of the Code. If a U.S. Holder desires to file such a waiver, the U.S. Holder should consult his or her own tax advisor.

        Not Essentially Equivalent to a Dividend.    A purchase of Shares pursuant to the Offer will be treated as "not essentially equivalent to a dividend" if it results in a "meaningful reduction" in the selling U.S. Holder's proportionate interest in the Company. Whether a U.S. Holder meets this test will depend on relevant facts and circumstances. In measuring the change, if any, in a U.S. Holder's proportionate interest in the Company, the meaningful reduction test is applied by taking into account all Shares that the Company purchases pursuant to the Offer, including Shares purchased from other U.S. Holders.

        The Internal Revenue Service (the "IRS") has held in a published ruling that, under the particular facts of that ruling, a small reduction in the percentage share ownership of a small minority shareholder in a publicly and widely held corporation who did not exercise any control over corporate affairs constituted a "meaningful reduction". If, taking into account the constructive ownership rules of Section 318 of the Code, a U.S. Holder owns Shares that constitute only a minimal interest in the Company and does not exercise any control over the affairs of the Company, any reduction in the U.S. Holder's percentage ownership interest in the Company should be a "meaningful reduction". Such selling U.S. Holder should, under these circumstances, be entitled to treat his or her sale of Shares pursuant to the Offer as a sale for U.S. Federal income tax purposes. Shareholders should consult their own tax advisors with respect to the application of the "not essentially equivalent to a dividend" test in their particular circumstances.

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        Substantially Disproportionate.    A purchase of Shares pursuant to the Offer will be "substantially disproportionate" as to a U.S. Holder if (i) the percentage of the then outstanding Shares actually and constructively owned by such U.S. Holder immediately after the purchase is less than 80% of the percentage of the outstanding Shares actually and constructively owned by such U.S. Holder immediately before the purchase, and (ii) the percentage of the outstanding voting stock of the Company actually and constructively owned by such U.S. Holder immediately after the purchase is less than 80% of the percentage of the outstanding voting stock of the Company actually and constructively owned by such U.S. Holder immediately before the purchase. Shareholders should consult their own tax advisors with respect to the application of the "substantially disproportionate" test in their particular circumstances.

        The Company Cannot Predict Whether There Will Be Sale or Distribution Treatment.    The Company cannot predict whether or the extent to which the Offer will be oversubscribed. If the Offer is oversubscribed, proration of tenders pursuant to the Offer will cause the Company to accept fewer Shares than are tendered. Consequently, the Company can give no assurance that a sufficient number of any U.S. Holder's Shares will be purchased pursuant to the Offer to ensure that such purchase will be treated as a sale, rather than as a distribution, for U.S. Federal income tax purposes under the rules discussed above.

        Corporate Dividends-Received Deduction.    In the case of a corporate U.S. Holder, if cash received pursuant to the Offer is treated as a dividend, the resulting dividend income will not qualify for the dividends-received deduction otherwise generally available to corporate U.S. Holders.

        Consequences to Shareholders Who Do Not Sell Shares Pursuant to the Offer.    Shareholders (including Non-U.S. Holders) who do not sell Shares pursuant to the Offer will not incur any U.S. Federal income tax liability as a result of the consummation of the Offer.

        Taxation of Non-U.S. Holders.    Non-U.S. Holders will generally not be subject to U.S. Federal income taxation as a result of selling Shares pursuant to the Offer. The rules governing U.S. Federal income taxation of the receipt by Non-U.S. Holders of cash pursuant to the Offer, however, are complex and Non-U.S. Holders should consult their own tax advisors concerning the application of U.S. Federal, state, local and foreign income tax laws in their particular circumstances.

        Backup Federal Income Tax Withholding.    See Section 2 with respect to the application of backup United States Federal income tax withholding.

        To prevent backup withholding and possible penalties, each shareholder should complete the substitute Form W-9 included in the Letter of Transmittal. In order to qualify for an exemption from backup withholding, a Non-U.S. Holder must submit a properly executed IRS Form W-8BEN, W-8BEN-E or other applicable form to the Depositary. Any amount paid as backup withholding will be creditable against the shareholder's U.S. Federal income tax liability provided that the required information is timely furnished to the IRS.

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        7. Price Range of Shares; Dividends.    The Shares are listed and traded on the NYSE under the symbol "WTM" and the Bermuda Stock Exchange under the symbol "WTM-BH". The following table sets forth, for the quarters indicated, the high and low sales prices per Share as reported on the NYSE:

        On August 16, 2017, the last full trading day prior to the public announcement of the Offer, the last reported closing sale price per Share as reported on the NYSE was $850.65. SHAREHOLDERS ARE URGED TO OBTAIN A CURRENT MARKET QUOTATION FOR THE SHARES.

        Historically, dividends have been declared and paid on an annual basis. On March 1, 2017, the Board declared an annual dividend of $1.00 per Share, payable March 29, 2017, to shareholders of record as of March 20, 2017. There can be no assurance that we will pay a regular annual dividend in the future. The amount and timing of all future dividend payments is subject to the discretion of the Board and will depend upon business conditions, results of operations, the Company's financial condition and other factors.

        8. Conditions to the Offer.    Notwithstanding any other provision of the Offer, and in addition to (and not in limitation of) the Company's right to extend, amend or terminate the Offer as set forth in Section 14, the Company shall not be required to accept for payment or pay for any Shares tendered, or may postpone the acceptance for payment of or the payment for Shares tendered, subject to Rule 13e-4(f) promulgated under the Exchange Act, if, before acceptance for payment of or payment for any such Shares, any of the following has occurred:

•

there shall have been threatened, instituted or pending any action or proceeding by any government or governmental, regulatory or administrative agency or authority or tribunal or any other person, domestic or foreign, before any court or governmental, regulatory or administrative authority, agency or tribunal, domestic, foreign or supranational, that (i) directly or indirectly challenges, makes illegal, or delays or otherwise directly or indirectly restrains, prohibits or otherwise adversely affects the making of the Offer or the acquisition of Shares pursuant to the Offer, or otherwise relates in any manner to the Offer; or (ii) we reasonably determine does or could materially and adversely affect the business, condition (financial or otherwise), income, assets, operations or prospects of the Company and its subsidiaries or otherwise materially impair the contemplated future conduct of the business of the Company or any of its principal subsidiaries or materially impair the benefits of the Offer to the Company;



•

there shall have been any action threatened, pending or taken, or approval withheld, or any statute, rule, regulation, judgment, order or injunction threatened, proposed, sought,

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promulgated, enacted, entered, amended, enforced or deemed to be applicable to the Offer or the Company or any of its subsidiaries, by any court or any government or governmental, regulatory or administrative authority, agency or tribunal, domestic, foreign or supranational, that (i) directly or indirectly makes the acceptance for payment of, or payment for, some or all the Shares illegal or otherwise restricts or prohibits consummation of the Offer; (ii) directly or indirectly delays or restricts the ability of the Company, or renders the Company unable, to accept for payment, or pay for, some or all the Shares; or (iii) in our reasonable judgment does or could materially and adversely affect the business, condition (financial or otherwise), income, assets, operations or prospects of the Company and its subsidiaries or otherwise materially impair the contemplated future conduct of the business of the Company or any of its principal subsidiaries or materially impair the benefits of the Offer to the Company;

•

there shall have occurred (i) any general suspension of trading in, or limitation on prices for, securities on any United States national securities exchange or in the over-the-counter market (excluding any coordinated trading halt triggered solely as a result of a specified decrease in a market index); (ii) the declaration of a banking moratorium or any suspension of payments in respect of banks in the United States, the European Union or Bermuda; (iii) the commencement of a war, armed hostilities, an act of terrorism, or other international or national crisis directly or indirectly involving the United States, the European Union or Bermuda; (iv) any limitation (whether or not mandatory) by any governmental, regulatory or administrative agency or authority on, or any event that we reasonably determine might affect, the extension of credit by banks or other lending institutions in the United States, the European Union or Bermuda, (v) any change in the general political, market, economic or financial conditions in the United States or abroad that we reasonably determine does or could materially and adversely affect the business, condition (financial or otherwise), income, assets, operations or prospects of the Company and its subsidiaries or the trading in the Shares or otherwise materially impair the contemplated future conduct of the business of the Company or any of its principal subsidiaries or materially impair the benefits of the Offer to the Company; or (vi) in the case of any of the foregoing existing at the time of the commencement of the Offer, in our reasonable judgment, a material escalation, acceleration or worsening thereof;



•

there shall have occurred any decline of more than 10% in the market price for the Shares or in the Dow Jones Industrial Average, New York Stock Exchange Composite Index, NASDAQ Composite Index or the Standard and Poor's 500 Index, as measured in each case from the close of business on August 16, 2017;



•

there shall have occurred a material change in U.S. or any other currency exchange rates or a suspension of or limitation on the markets for such currencies that we reasonably determine does or could materially and adversely affect the business, condition (financial or otherwise), income, assets, operations or prospects of the Company or any of its subsidiaries or materially impair the future conduct of the business of the Company or any of its principal subsidiaries or materially impair the benefits of the Offer to the Company;



•

after August 16, 2017, any other change, event or development shall occur or be threatened or be reasonably anticipated by the Company that we reasonably determine does or could materially and adversely affect the business, condition (financial or otherwise), income, assets, operations or prospects of the Company and its subsidiaries or otherwise materially impair the contemplated future conduct of the business of the Company or any of its principal subsidiaries or materially impair the benefit of the Offer to the Company;



•

legislation amending the Code (as defined in Section 6) shall have been passed by either the U.S. House of Representatives or the Senate or pending before the U.S. House of Representatives or the Senate or any committee thereof that we reasonably determine could

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materially and adversely change the tax consequences of the transaction contemplated by the Offer to the Company or any of its subsidiaries;

•

any person, entity or group shall have filed a Notification and Report Form under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 (the "HSR Act"), or made a public announcement reflecting an intent to acquire the Company or any of its subsidiaries or a material portion of their respective assets or securities;



•

after August 16, 2017, any tender or exchange offer with respect to the Shares (other than the Offer), or any merger, acquisition, business combination or other similar transaction with or involving the Company or any of its subsidiaries, shall have been proposed, announced or made by any person or entity;



•

we reasonably determine that the purchase of Shares pursuant to the Offer will cause either (i) the Shares to be held of record by less than 300 persons; (ii) delisted from the NYSE or (iii) eligible for deregistration under the Exchange Act; or



•

after August 16, 2017, any event, change or other development shall occur that we reasonably determine makes it likely that a condition to the OneBeacon Transaction will not be satisfied or waived. Such conditions are specified in Annex A.

        The foregoing conditions are for the sole benefit of the Company and may be asserted by us regardless of the circumstances (including any action or inaction by the Company) giving rise to any such condition and any such condition may be waived by us, in whole or in part, at any time and from time to time in our sole discretion. In certain cases, waiver of a condition to the Offer would require an extension of the Offer. See "Section 14. Extension of the Offer; Termination; Amendment".

        The Company's failure at any time to exercise any of the foregoing rights shall not be deemed a waiver of any such right; the waiver of any such right with respect to particular facts and circumstances shall not be deemed a waiver with respect to any other facts or circumstances; and each such right shall be deemed an ongoing right which may be asserted at any time and from time to time. At or before the expiration of the Offer, all of the conditions to the Offer, other than those requiring necessary government approval, must have been satisfied or waived. Any determination by the Company concerning the events described above will be final and binding on all parties. Notwithstanding the foregoing, in the event that one or more of the events described above occurs, we will promptly notify shareholders of our determination as to whether to: (i) waive or modify the applicable condition(s) and continue the Offer; or (ii) terminate the Offer.

        9. Certain Information Concerning the Company.    White Mountains is an exempted Bermuda limited liability company whose principal businesses are conducted through its property and casualty insurance and reinsurance subsidiaries and affiliates. The Company's headquarters are located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11.

        Where You Can Find More Information.    The Company is subject to the informational filing requirements of the Exchange Act and, in accordance therewith, is required to file periodic reports, proxy statements and other information with the SEC relating to its business, financial condition and other matters. Such reports, proxy statements and other information can be inspected and copied at the SEC's public reference rooms in Washington, D.C. at 100 F Street, N.E., Washington, D.C. 20549. Information regarding the public reference facilities may be obtained from the SEC by telephoning 1-800-SEC-0330. The Company's filings are also available to the public on the SEC Internet site (http://www.sec.gov).

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        Incorporation by Reference.    The rules of the SEC allow us to "incorporate by reference" information into this document, which means that we can disclose important information to you by referring you to another document filed separately with the SEC. The following documents contain important information about us and we incorporate them by reference:

•

Annual Report on Form 10-K for the fiscal year ended December 31, 2016, filed on February 27, 2017;



•

Quarterly Reports on Form 10-Q for the quarter ended March 31, 2017, filed on May 2, 2017 and for the quarter ended June 30, 2017, filed on August 4, 2017;



•

Current Reports on Form 8-K filed on March 6, 2017, May 2, 2017, May 8, 2017, May 31, 2017, June 30, 2017, and August 4, 2017; and



•

Definitive Proxy Statement for our 2017 annual meeting of shareholders, filed on April 19, 2017.

        You can obtain any of the documents incorporated by reference in this document from the SEC's website at the address described above. You may also request a copy of these filings, at no cost, by writing or telephoning the Information Agent at its address and telephone number set forth below.

        10. Source and Amount of Funds.    Assuming that the maximum number of Shares are tendered in the Offer and the Purchase Price is an amount between $825 and $875 per Share, the aggregate purchase price for the Shares will be between approximately $412.5 million and $437.5 million. The Company intends to fund the purchase of Shares with available cash, including cash from the sale of liquid investments. We do not currently have any alternative financing arrangements or plans that would fund the purchase of Shares in the Offer in the event that we do not have sufficient available cash. We may also draw on the revolving credit facility (as defined below) to fund the purchase of Shares.

        On August 14, 2013, the Company entered into a revolving credit facility, as amended on September 24, 2014, with a syndicate of lenders administered by Wells Fargo Bank, N.A., which has a total commitment of $425.0 million and has a maturity date of August 14, 2018 (as amended, the "revolving credit facility"). As of August 16, 2017, the revolving credit facility was undrawn. Amounts drawn under the revolving credit facility that are base rate loans generally bear interest at the highest of (1) the federal funds flow rate plus 0.5%, (2) Wells Fargo Bank's "prime rate" and (3) the eurodollar rate (as defined in the revolving credit facility) plus 1.0%, plus, in each case, the applicable margin specified in the revolving credit facility (which ranges from 0.2% to 0.9% depending on the Company's total consolidated debt to total consolidated capitalization ratio). Amounts drawn under the revolving credit facility that are eurodollar loans generally bear interest at LIBOR plus the applicable margin specified in the revolving credit facility (which ranges from 1.2% to 1.9% depending on the Company's total consolidated debt to total consolidated capitalization ratio). The revolving credit facility contains various affirmative, negative and financial covenants which we consider to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. This summary of the revolving credit facility does not purport to be complete and is qualified in its entirety by reference to the revolving credit facility filed as an exhibit to the Company's Quarterly Report on Form 10-Q dated October 28, 2013 and the amendment thereto filed as an exhibit to the Company's Quarterly Report on Form 10-Q dated October 29, 2014, and is incorporated by reference herein.

        11. Interests of Directors and Executive Officers; Transactions and Arrangements Concerning the Shares.    As of August 16, 2017, we had 4,336,625 issued and outstanding Shares. The 500,000 Shares that the Company is offering to purchase pursuant to the Offer represent approximately 12% of the Shares outstanding as of August 16, 2017.

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        As of August 16, 2017, our directors and executive officers as a group of 12 persons beneficially owned an aggregate of approximately 99,408 Shares (including Shares that are subject to the proxy described above), representing approximately 2.3% of the total number of outstanding Shares. All of our directors and executive officers have advised us that they do not intend to tender any of their Shares in the Offer. Accordingly, assuming we purchase 500,000 Shares in the Offer, the proportional holdings of our directors and executive officers will increase to approximately 2.6%. The Trust under the Executive Survivor Income Plan maintained by our subsidiary, OneBeacon Services, LLC, may elect to tender a number of Shares representing up to an aggregate value of $4.0 million. There can be no assurances that such Trust will tender Shares in the Offer or the price or prices at which any such Shares may be tendered. However, our directors and executive officers may, in compliance with applicable law, sell their Shares in open market transactions at prices that may or may not be more favorable than the Purchase Price to be paid to our shareholders in the Offer.

        Schedule I provides information with respect to the beneficial ownership of our Common Shares by (i) each of our directors, (ii) each of our executive officers and (iii) all directors and executive officers as a group. We based the share amounts on each person's beneficial ownership of our Common Shares as of August 16, 2017.

        Beneficial ownership is determined in accordance with the rules of the SEC. In computing the number of Shares beneficially owned by a person and the percentage of ownership held by that person, Shares subject to options or which are otherwise subject to vesting and that are currently exercisable or exercisable within 60 days of the date of this Offer to Purchase are deemed outstanding. These Shares, however, are not deemed outstanding for purposes of computing percentage ownership of each other shareholder.

        Based on our records and on information provided to us by our directors, executive officers, affiliates and subsidiaries, neither we nor our directors, executive officers, affiliates or subsidiaries have effected any transactions in the Shares during the 60 days prior to the date hereof, except as follows:

•

On July 13, 2017, the Company purchased 235,000 Shares from Franklin Mutual Advisers, LLC at a price of $850 per Share.

Long-Term Incentive Plan

        White Mountains's Long-Term Incentive Plan (the "LTIP") provides for grants of various types of share-based and non-share-based incentive awards to our key employees and service providers, including stock options, restricted shares, performance shares and performance units. The LTIP as adopted by the Board, was approved by the Company's sole shareholder in 1985 and was subsequently amended by its shareholders in 1995, 2001, 2003, 2005, 2010 and 2013. The compensation committee of the Board (the "Compensation Committee") administers the LTIP and has the authority to select employees, directors and consultants to receive awards under the LTIP, determine the type, size and terms of awards granted under the LTIP, and interpret the LTIP and related awards.

        The LTIP provides that up to 250,000 common shares could be granted "at target" under the LTIP with up to 500,000 common shares actually issuable if all grants were made as performance share grants that paid out at 200% of target and were settled with common shares. The maximum amount of performance shares that can be granted to a participant pursuant to an award of performance shares with respect to any particular award period of one year or more cannot exceed 25,000 common shares. The maximum amount of compensation that can be earned by a participant with respect to a grant of performance units made in any single year cannot exceed $25,000,000. A participant cannot receive options or stock appreciation rights on more than 50,000 common shares during any one year. In the event of certain events effecting the capitalization of the Company, such as a recapitalization or stock split, the maximum number of Shares available for issuance under the LTIP, the individual limits described above, the number of Shares underlying outstanding awards and the exercise price applicable to outstanding awards may be equitably adjusted as determined by the Compensation Committee.

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        White Mountains takes a more conservative approach in managing the LTIP than required by the NYSE by counting the target number of shares granted (whether or not ultimately earned or, if earned, paid in common shares). Based on this approach, as of April 19, 2017, White Mountains actually has issued 151,479 Shares, meaning that 98,521 Shares remained available for future grants at target under the LTIP, which could result in up to 197,042 Shares being issued.

        Pursuant to the LTIP, the Compensation Committee has typically structured our long-term incentive compensation as performance shares and restricted shares. Performance shares are awards of phantom common shares some or all of which are earned if performance goals established by the Compensation Committee are satisfied over a specified award period, usually three years. Based on the level of performance against established goals, the number of performance shares earned can range from 0% to 200% of the number of target shares originally granted. Performance shares are typically paid in cash, though all or a portion may be settled in common shares at the election of the Compensation Committee. Awards of performance shares are generally forfeited if any participant terminates employment with the Company and its subsidiaries prior to the end of the award period for any reason other than death, disability or retirement, or pursuant to a trigger event following a change in control. Restricted shares are Shares that may not be sold, assigned, transferred, pledged, hypothecated or otherwise disposed of during a period designated by the Compensation Committee. The Company may repurchase the restricted stock at the price specified by the Compensation Committee at the time of grant if the holder's employment with the Company terminates before the end of the applicable restricted period for a reason other than death, disability or, in the discretion of the Board, retirement or other voluntary termination, or pursuant to a trigger event following a change in control.

        As of August 16, 2017, our executive officers held the following number of performance shares and restricted shares (with performance shares calculated based on target performance): Reid T. Campbell—7,250 performance shares and 7,250 restricted shares; J. Brian Palmer—1,820 performance shares and 1,820 restricted shares; David B. Linker—4,725 performance shares and 4,725 restricted shares; T. Michael Miller—none; G. Manning Rountree—8,800 performance shares and 8,800 restricted shares; Robert L. Seelig—6,500 performance shares and 6,500 restricted shares; and all executive officers as a group—29,095 performance shares and 29,095 restricted shares. Mr. Miller receives long-term incentive compensation awards with respect to securities of our subsidiary, OneBeacon Insurance Group, Ltd ("OneBeacon"). Our non-employee directors do not hold any awards subject to vesting under the LTIP. See below for further discussion on the compensation arrangements for our directors.

Director Compensation

        Management directors do not receive any additional compensation for their role as a director. As of May 26, 2016, meeting fees for non-management directors were eliminated and the annual retainer was increased to $135,000. Additional retainers in the following amounts are provided to those directors serving in the following roles: Chairman of the Audit Committee ($100,000), Deputy Chairman ($50,000), Chairman of any other Board committee ($25,000) and members of the Audit Committee ($15,000). Retainers were paid in cash except for Mr. Brouillette who received an equivalent value in Shares.

        On May 26, 2016, all non-management directors received an annual grant of 250 Shares, except for Mr. Davis who received 300 Shares for his role as Deputy Chairman of the Board. Mr. Brouillette received additional Shares for payment of his retainer. All Shares issued were valued at $808.49 per share, the market price on the date the Shares were granted.

        Directors who serve as directors of the Company's subsidiaries or affiliates typically receive additional compensation for their service. Mr. Davis received $89,000 in compensation as a director of OneBeacon, $60,000 in compensation as the Company's representative on the Board of Compare.com,

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$40,000 in compensation as a director of MediaAlpha. Mr. Smith received $278,500 in director compensation as the Chairman of OneBeacon.

Other Arrangements

        White Mountains entered into new arrangements in connection with the recent departures of two of our former executive officers, Ray Barrette and David T. Foy. Mr. Barrette retired as Chairman and CEO on March 1, 2017. In connection with his retirement, as more fully described in the White Mountains's Current Report on Form 8-K dated March 1, 2017, White Mountains entered into a letter agreement with Mr. Barrette pursuant to which it agreed to pay Mr. Barrette (i) a cash bonus for 2017 equal to $187,500 and (ii) a cash retirement payment equal to approximately $21 million. Mr. Barrette's unvested long-term performance shares and restricted shares were canceled in connection with his retirement. The letter agreement also provides that Mr. Barrette will be subject to certain restrictive covenants regarding competition, solicitation and disparagement that apply through December 31, 2018. As of March 1, 2017, Mr. Barrette had pledged 35,952 Shares as collateral for a credit line of up to $15 million.

        On May 2, 2017, in connection with Mr. Foy's agreeing to become an advisor to senior management after his departure as our Executive Vice President and Chief Financial Officer, we entered into an employment agreement with Mr. Foy. Under the employment agreement, Mr. Foy will (A) be paid a base salary at the annual rate of $500,000 and (B) continue to participate in White Mountains's employee benefit plans. In addition, White Mountains will pay Mr. Foy a cash bonus for White Mountains's 2017 fiscal year equal to his 2017 target bonus amount multiplied by the 2017 bonus pool percentage generally applicable to White Mountains's employees with respect to White Mountains's 2017 bonus program. Further, in exchange for a mutual release of claims, (i) Mr. Foy's unvested performance shares granted under the LTIP were canceled and cashed out based on the fair market value of the shares on the effective date of the release and (ii) Mr. Foy's 5,900 restricted shares granted under the LTIP were vested on the effective date of the release. The employment agreement also provides that Mr. Foy will be subject to certain restrictive covenants.

        We have no employment or change-in-control arrangements with our current executive officers. Our executive officers generally receive an annual base salary and are eligible to receive an annual bonus based on the performance of his respective business unit. The aggregate bonus pool size for each business unit could range from 0% to 200% of target, depending on performance. Individual bonuses can vary widely around the pool average based on individual performance and no cap (other than the size of the pool) applies to any single individual. Further, at White Mountains, severance benefits to our executive officers, if any, are determined by the Compensation Committee in its sole discretion. At our operating subsidiaries, our executive officers participate in the severance plans, if any, generally applicable to such officers at those companies.

        Our executive officers are generally eligible to participate in the OneBeacon 401(k) Plan. The OneBeacon 401(k) Plan contains cash deferred arrangements. Participants in the OneBeacon 401(k) Plan may direct the trustee of the OneBeacon 401(k) Plan to invest funds in any investment option available under the OneBeacon 401(k) Plan. The OneBeacon 401(k) Plan provides employees with a menu of investment options, including Shares. In addition, certain of our executive officers are also participants in frozen nonqualified pension and savings plans of OneBeacon. Our executive officers also participate in our other benefit plans on the same terms as our other employees, and are eligible for limited perquisites.

        12. Certain Legal Matters.    The Company is not aware of any license or regulatory permit that appears to be material to its business that might be adversely affected by its acquisition of Shares as contemplated in the Offer or of any approval or other action by any government or governmental, administrative or regulatory authority or agency, domestic, foreign or supranational, that would be

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required for the Company's acquisition or ownership of Shares pursuant to the Offer. Should any such approval or other action be required, the Company currently contemplates that it will seek such approval or other action. The Company cannot predict whether it may determine that it is required to delay the acceptance for payment of Shares tendered pursuant to the Offer pending the outcome of any such matter. There can be no assurance that any such approval or other action, if needed, would be obtained or would be obtained without substantial conditions or that the failure to obtain any such approval or other action might not result in adverse consequences to the Company's business. The Company intends to make all required filings under the Exchange Act.

        As of the date of this Offer to Purchase, there are no material legal proceedings relating to the Offer.

        The Company's obligation under the Offer to accept Shares for payment is subject to certain conditions. See "Section 8. Conditions to the Offer".

        13. Effects of the Offer on the Market for Shares; Registration Under the Exchange Act.    The purchase by the Company of Shares under the Offer will reduce our "public float" (the number of Shares owned by non-affiliate shareholders and available for trading in the securities markets). This reduction in our public float may result in lower stock prices and/or reduced liquidity in the trading market for the Shares following completion of the Offer. In addition, the Offer may reduce the number of White Mountains shareholders. As of August 16, 2017, we had 4,336,625 issued and outstanding Shares. Shareholders may be able to sell non-tendered Shares in the future on the NYSE or otherwise, at a net price higher or lower than the Purchase Price in the Offer. We can give no assurance, however, as to the price at which a shareholder may be able to sell such Shares in the future.

        We anticipate that there will be a sufficient number of Shares outstanding and publicly traded following completion of the Offer to ensure a continued trading market for such Shares. Based upon published guidelines of the NYSE, we do not believe that our purchase of Shares under the Offer will cause the remaining outstanding Shares to be delisted from the NYSE.

        The Shares are currently "margin securities" under the rules of the Federal Reserve Board. This classification has the effect, among other things, of allowing brokers to extend credit to their customers using the Shares as collateral. The Company believes that, following the purchase of Shares under the Offer, the Shares remaining outstanding will continue to be margin securities for purposes of the Federal Reserve Board's margin rules and regulations.

        The Shares are registered under the Exchange Act, which requires, among other things, that we furnish certain information to our shareholders and the SEC and comply with the SEC's proxy rules in connection with meetings of our shareholders. We believe that our purchase of Shares under the Offer will not result in the Shares becoming eligible for deregistration under the Exchange Act.

        14. Extension of the Offer; Termination; Amendment.    Subject to the terms and conditions of the Offer, the Company expressly reserves the right, in its sole discretion, at any time and from time to time, and regardless of whether or not any of the events set forth in "Section 8. Conditions to the Offer" shall have occurred or shall be deemed by the Company to have occurred, to extend the period of time during which the Offer is open and thereby delay acceptance for payment of any Shares by giving oral or written notice of such extension to the Depositary and making a public announcement thereof. During any such extension, all Shares previously tendered and not purchased or withdrawn will remain subject to the Offer, except to the extent that such Shares may be withdrawn as set forth in "Section 3. Withdrawal Rights".

        The Company also expressly reserves the right, in its sole discretion, to withdraw or terminate the Offer and not accept for payment or pay for any Shares not theretofore accepted for payment or paid for or, subject to applicable law, to postpone payment for Shares upon the occurrence of any of the conditions specified in Section 8 by giving oral or written notice of the termination or postponement to

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the Depositary and making a public announcement of the termination or postponement. Additionally, the Company expressly reserves the right, subject to applicable law, to postpone payment for Shares under circumstances including but not limited to the occurrence of any of the conditions specified in "Section 8. Conditions to the Offer" by notifying the Depositary and making a public announcement thereof. However, the ability of the Company to delay the payment for Shares which the Company has accepted for payment is limited by Rule 13e-4(f)(5) promulgated under the Exchange Act, which requires that bidder pay the consideration offered or return the securities deposited by, or on behalf of, holders of securities promptly after the termination or withdrawal of the Offer.

        If the Company is delayed in its payment for the Shares or is unable to pay for the Shares pursuant to the Offer for any reason, then, without prejudice to the Company's rights under the Offer, the Depositary may retain tendered Shares on behalf of the Company, and such Shares may not be withdrawn except to the extent tendering shareholders are entitled to withdrawal rights as described in "Section 3. Withdrawal Rights".

        Subject to compliance with applicable law, the Company further reserves the right, in its sole discretion, and regardless of whether or not any of the events set forth in "Section 8. Conditions to the Offer" shall have occurred or shall be deemed by the Company to have occurred, to amend the Offer in any respect (including by decreasing or increasing the consideration offered in the Offer or by increasing or decreasing the number of Shares being sought in the Offer). Amendments to the Offer may be made at any time, effected by public announcement thereof. Such announcement, in the case of an extension, is to be issued no later than 9:00 a.m., New York City time, on the next business day after the previously scheduled Expiration Time. Any disclosure of a material change in the information published, sent or given to shareholders will be disseminated promptly to shareholders in a manner reasonably calculated to inform shareholders of such change to the extent required by Rules 13e-4(d)(2) and 13e-4(e)(3) promulgated under the Exchange Act. Without limiting the manner in which the Company may choose to make a public announcement pursuant to or concerning the Offer, except as required by applicable law, the Company shall have no obligation to publish, advertise or otherwise communicate any such public announcement other than by making a release to the Wall Street Journal, the New York Times or another comparable service.

        If the Company makes a material change in the terms of the Offer or the information concerning the Offer, or if it waives a material condition of the Offer, the Company will extend the Offer to the extent required by Rules 13e-4(e)(3) and 13e-4(f)(1) promulgated under the Exchange Act. If we make any change to the purchase price range at which we are offering to purchase Shares in the Offer, decrease the number of Shares purchasable in the Offer or increase the number of Shares sought in the Offer by an amount exceeding 2% of our outstanding Shares, the Offer must remain open a minimum of ten business days from and including the date such change is first published, sent or given to shareholders in the manner specified in this Section 14. The minimum period during which the Offer must remain open following material changes in the terms of the Offer or information concerning the Offer, other than a change in price or change in percentage of Shares sought, will depend upon the facts and circumstances then existing, including the relative materiality of the changed terms or information. In a public release, the SEC has stated its views that an offer must remain open for a minimum period of time following a material change in the terms of the Offer and that waiver of a material condition is a material change in the terms of the Offer. The release states that an offer should remain open for a minimum of five business days from the date a material change is first published or sent or given to security holders and that, if material changes are made with respect to information not materially less significant than the offer price and the number of Shares being sought, a minimum of 10 business days may be required to allow for adequate dissemination to shareholders and investor response. For purposes of the Offer, a "business day" means any day other than a Saturday, Sunday or United States Federal holiday and consists of the time period from 12:01 a.m. through 12:00 midnight, New York City time. The requirements to extend the Offer do not apply to the

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extent that the number of business days remaining between the occurrence of the change and the then-scheduled Expiration Time equals or exceeds the minimum extension period that would be required because of such amendment.

        15. Fees and Expenses.    Except as set forth below, the Company will not pay any fees or commissions to any broker, dealer or other person for soliciting tenders of Shares pursuant to the Offer.

        We have retained Barclays Capital Inc. and J.P. Morgan Securities LLC severally to act as the Dealer Managers in connection with the Offer. In their roles as such, the Dealer Managers may contact brokers, dealers and similar entities and may provide information regarding the Offer to those that it contacts or persons that contact it. The Dealer Managers will receive reasonable and customary compensation for their services in connection with the Offer. The Company has also agreed to reimburse the Dealer Managers for certain out-of-pocket expenses and to indemnify the Dealer Managers against certain liabilities which could occur in connection with the Offer, including certain liabilities under the federal securities laws. The Dealer Managers have provided in the past, and are currently providing, various investment banking and other services to us, for which services they receive customary compensation from us. In particular, an affiliate of Barclays Capital Inc. is an agent and lender under our revolving credit facility. We expect the Dealer Managers and their respective affiliates to continue to render such services, for which they would expect to continue to receive customary compensation from us. Additionally, in the ordinary course of business, including in its trading and brokerage operations and in a fiduciary capacity, the Dealer Managers and their respective affiliates may hold positions, both long and short, for their own accounts and for those of their customers, in our securities, including the Shares, and/or other financial instruments.

        We have also retained D.F. King to act as Information Agent in connection with the Offer. The Information Agent will assist shareholders who request assistance in connection with the Offer and may request brokers, dealers and other nominee shareholders to forward material relating to the Offer to beneficial owners for which they act as nominees. D.F. King will receive reasonable and customary compensation for its services in connection with the Offer and will be reimbursed for reasonable expenses, including the reasonable fees and expenses of counsel. The Company has agreed to indemnify D.F. King against certain liabilities which could occur in connection with the Offer, including certain liabilities under the federal securities laws. D.F. King has not been retained and is not authorized to make solicitations or recommendations in connection with the Offer in its role as Information Agent.

        We have also retained Computershare to act as Depositary in connection with the Offer. Computershare, in its capacity as Depositary, will receive reasonable and customary compensation for its services in connection with the Offer and will be reimbursed by us for reasonable expenses, including reasonable fees and expenses of counsel. The Company has agreed to indemnify Computershare against certain liabilities which could occur in connection with the Offer, including liabilities under the federal securities laws.

        The Company will not pay any fees or commissions to any broker, dealer, commercial bank, trust company or other person (other than fees to the Dealer Managers, the Information Agent and Depositary), for soliciting Shares pursuant to the Offer. The Company will, however, on request, reimburse such persons for customary handling and mailing expenses incurred in forwarding materials in respect of the Offer to the beneficial owners for which they act as nominees. No broker, dealer, commercial bank or trust company has been authorized to act as an agent for the Company, the Dealer Managers, the Information Agent or the Depositary for the purpose of the Offer. The Company will not pay (or cause to be paid) any share transfer taxes on its purchase of Shares pursuant to the Offer, except as otherwise provided in this Offer to Purchase or in Instruction 6 of the Letter of Transmittal.

        Certain officers and employees of the Company may render services in connection with the Offer, but they will not receive any additional compensation for such services.

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        16. Miscellaneous.    The Offer is being made to all holders of Shares. The Company is not aware of any U.S. State where the making of the Offer is prohibited by administrative or judicial action pursuant to any valid state statute. If the Company becomes aware of any valid state statute prohibiting the making of the Offer or the acceptance of Shares pursuant thereto, the Company will make a good faith effort to comply with any such state statute or seek to have such statute declared inapplicable to the Offer. If, after such good faith effort, the Company cannot comply with any such state statute, the Offer will not be made to (nor will tenders be accepted from or on behalf of) the holders of Shares in such state. In any jurisdiction where the securities, blue sky or other laws require the Offer to be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of the Company by one or more registered brokers or dealers licensed under the laws of such jurisdiction.

        OUR BOARD HAS APPROVED THE OFFER. HOWEVER, NONE OF THE COMPANY, OUR BOARD, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT MAKES ANY RECOMMENDATION AS TO WHETHER ANY SHAREHOLDER SHOULD TENDER OR REFRAIN FROM TENDERING SHARES PURSUANT TO THE OFFER, OR AS TO THE PRICE OR PRICES AT WHICH A SHAREHOLDER MAY CHOOSE TO TENDER SHARES. EACH SHAREHOLDER MUST MAKE HIS OR HER OWN DECISION AFTER CONSULTING WITH HIS OR HER OWN ADVISORS WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO TENDER AND THE PRICE OR PRICES AT WHICH SUCH SHAREHOLDER WILL TENDER THEM. IN DOING SO, SHAREHOLDERS SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE AND IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING THE PURPOSES AND EFFECTS OF THE OFFER. SEE "SECTION 5. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER". YOU SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THIS OFFER TO PURCHASE OR IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING OUR REASONS FOR MAKING THE OFFER. SEE "SECTION 5. PURPOSE OF THE OFFER; CERTAIN EFFECTS OF THE OFFER".

        NO PERSON HAS BEEN AUTHORIZED TO GIVE ANY INFORMATION OR TO MAKE ANY REPRESENTATIONS IN CONNECTION WITH THE OFFER OTHER THAN THOSE CONTAINED IN THIS OFFER TO PURCHASE OR IN THE LETTER OF TRANSMITTAL. IF MADE OR GIVEN, SUCH RECOMMENDATION AND SUCH INFORMATION AND REPRESENTATIONS MUST NOT BE RELIED UPON AS HAVING BEEN AUTHORIZED BY THE COMPANY, OUR BOARD OF DIRECTORS, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT.

        The Company has filed with the SEC a Tender Offer Statement on Schedule TO, together with all exhibits thereto, pursuant to Rule 13e-4 promulgated under the Exchange Act, furnishing certain additional information with respect to the Offer. Such Schedule and any amendments thereto, including exhibits, may be inspected and copies may be obtained from the offices of the SEC in the manner set forth in "Section 9. Certain Information Concerning the Company" (except that they will not be available at the regional offices of the SEC).

White Mountains Insurance Group, Ltd.

August 17, 2017

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SCHEDULE I

INFORMATION CONCERNING THE DIRECTORS AND EXECUTIVE
OFFICERS OF THE COMPANY

        Directors and Executive Officers of the Company.    Set forth in the table below are the names, titles, business addresses, telephone numbers and beneficial ownership of Shares information of the directors and executive officers of the Company. Each person identified below, except for the Company and Yves Brouillette (Canadian citizen), is a United States citizen.

        The address and telephone number of each person listed is c/o White Mountains Insurance Group, Ltd., 80 South Main Street, Hanover, New Hampshire 03755, telephone (603) 640-2200.

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ANNEX A

CONDITIONS PRECEDENT TO CLOSING OF THE ONEBEACON TRANSACTION

1.

(i) Any waiting period (or extension thereof) applicable to the OneBeacon Transaction under the HSR Act shall have been terminated or shall have expired and (ii) the consents of, or declarations, notifications or filings with, and the other terminations or expirations of waiting periods required from, the governmental authorities set forth in Section 6.01(b) of the Company Disclosure Letter (as such term is defined in the Merger Agreement) shall have been filed, have occurred or been obtained (clauses (i) and (ii), collectively, the "Required Regulatory Approvals"), and all such Required Regulatory Approvals shall be in full force and effect.

2.

No injunction, judgment or ruling enacted, promulgated, issued, entered, amended or enforced by any governmental authority (in each case, if with respect to any antitrust laws or insurance laws, solely with respect to the Required Regulatory Approvals) shall be in effect enjoining, restraining or otherwise making illegal or prohibiting consummation of the OneBeacon Transaction.

3.

The representations and warranties of OneBeacon (i) set forth in Section 3.06(b) of the Merger Agreement shall be true and correct in all respects as of the date of the Merger Agreement and as of the Closing Date (as such term is defined in the Merger Agreement) as though made as of the Closing Date, (ii) set forth in Section 3.02(a) of the Merger Agreement shall be true and correct in all respects other than de minimis inaccuracies therein, (iii) set forth in Section 3.02(b), Section 3.03(a), Section 3.03(b), Section 3.03(d), Section 3.14 and Section 3.24 of the Merger Agreement shall be true and correct in all material respects as of the date of the Merger Agreement and as of the Closing Date with the same effect as though made as of the Closing Date (except to the extent expressly made as of an earlier date, in which case as of such date) and (iv) set forth in the Merger Agreement, other than those Sections specifically identified in clause (i), (ii) or (iii) of this bullet, shall be true and correct (disregarding all qualifications or limitations as to "materiality", "Material Adverse Effect" and words of similar import set forth therein) as of the date of the Merger Agreement and as of the Closing Date with the same effect as though made as of the Closing Date (except to the extent expressly made as of an earlier date, in which case as of such date), except, in the case of this clause (iv), where the failure to be true and correct would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect (as such term is defined in the Merger Agreement). Intact shall have received a certificate signed on behalf of OneBeacon by an executive officer of OneBeacon to such effect.

4.

OneBeacon shall have performed or complied in all material respects with the obligations and agreements required to be performed or complied with by it under the Merger Agreement at or prior to the Effective Time (as such term is defined in the Merger Agreement), and Intact shall have received a certificate signed on behalf of OneBeacon by an executive officer of OneBeacon to such effect.

5.

The representations and warranties of Intact, Holdco (as such term is defined in the Merger Agreement) and Merger Sub (as such term is defined in the Merger Agreement) (i) set forth in Section 4.02(a), Section 4.02(b) and Section 4.02(d) of the Merger Agreement shall be true and correct in all material respects as of the date of the Merger Agreement and as of the Closing Date with the same effect as though made as of the Closing Date (except to the extent expressly made as of an earlier date, in which case as of such date) and (ii) set forth in the Merger Agreement, other than those Sections specifically identified in clause (i) of this bullet, shall be true and correct (disregarding all qualifications or limitations as to "materiality", "Parent Material Adverse Effect" and words of similar import set forth therein) as of the date of the Merger Agreement and as of the Closing Date with the same effect as though made as of the Closing Date (except to the extent expressly made as of an earlier date, in which case as of such date), except, in the case of this clause (ii), where the failure to be true and correct would not, individually or in the aggregate,

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reasonably be expected to have a Parent Material Adverse Effect (as such term is defined in the Merger Agreement). The Company shall have received a certificate signed on behalf of Intact, Holdco and Merger Sub by an executive officer of Intact to such effect.

6.

Intact, Holdco and Merger Sub shall have performed or complied in all material respects with the obligations and agreements required to be performed or complied with by them under the Merger Agreement at or prior to the Effective Time, and the Company shall have received a certificate signed on behalf of Intact, Holdco and Merger Sub by an executive officer of Intact to such effect.

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The Depositary for the Offer is:

Computershare Trust Company, N.A.

        DELIVERY OF THE LETTER OF TRANSMITTAL TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID BINDING DELIVERY TO THE DEPOSITARY.

        Any questions or requests for assistance or for additional copies of this Offer to Purchase, the Letter of Transmittal or the Notice of Guaranteed Delivery may be directed to the Information Agent. Shareholders may also contact their broker, dealer, commercial bank, trust company or other nominee for assistance concerning the Offer.

The Information Agent for the Offer is:
D.F. King & Co., Inc.
48 Wall Street, 22nd Floor
New York, NY 10005

Banks and Brokerage Firms Please Call Collect: (212) 269-5550

All Others Call Toll Free: (800) 893-5865

Email: wtm@dfking.com

The Dealer Managers for the Offer are:

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Exhibit (a)(1)(B)

LETTER OF TRANSMITTAL

To Tender Common Shares
of
WHITE MOUNTAINS INSURANCE GROUP, LTD.
Pursuant to the Offer to Purchase
Dated August 17, 2017

of up to 500,000 of its Common Shares
at a Purchase Price not greater than $875
or less than $825 per share.

The Depositary for the Offer is:

Computershare Trust Company, N.A.

        YOU SHOULD READ CAREFULLY THIS LETTER OF TRANSMITTAL, INCLUDING THE ACCOMPANYING INSTRUCTIONS, BEFORE YOU COMPLETE IT. FOR THIS LETTER OF TRANSMITTAL TO BE VALIDLY DELIVERED, IT MUST BE RECEIVED BY THE DEPOSITARY AT ONE OF THE ABOVE ADDRESSES BEFORE OUR OFFER EXPIRES (IN ADDITION TO THE OTHER REQUIREMENTS DETAILED IN THIS LETTER OF TRANSMITTAL AND ITS INSTRUCTIONS). DELIVERY OF THIS INSTRUMENT TO AN ADDRESS OTHER THAN THOSE SHOWN ABOVE DOES NOT CONSTITUTE A VALID DELIVERY. DELIVERIES TO THE COMPANY, THE INFORMATION AGENT, THE DEALER MANAGERS OR THE DEPOSITORY TRUST COMPANY ("DTC") WILL NOT BE FORWARDED TO THE DEPOSITARY AND WILL NOT CONSTITUTE A VALID DELIVERY.

        Indicate below the order (by certificate number) in which Shares are to be purchased in the event of proration (attach additional signed list if necessary). If you do not designate an order and if less than all Shares tendered are purchased due to proration, Shares will be selected for purchase by the Depositary. See Instruction 15.

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        This Letter of Transmittal is to be used only (a) if you desire to effect the tender transaction yourself, (b) if you intend to request your broker, dealer, commercial bank, trust company or other nominee to effect the transaction for you and the common shares of White Mountains Insurance Group, Ltd. (the "Company"), par value $1.00 per share (the "Shares"), are not registered in the name of such broker, dealer, commercial bank, trust company or other nominee or (c) by a broker, dealer, commercial bank, trust company or other nominee effecting the transaction as a registered owner or on behalf of a registered owner. A properly completed and duly executed Letter of Transmittal (or photocopy thereof bearing original signature(s) and any required signature guarantees), any certificates representing Shares tendered and any other documents required by this Letter of Transmittal should be mailed, by express or overnight delivery, or delivered to the Depositary at the appropriate address set forth herein and must be received by the Depositary prior to 12:00 midnight, New York City time, at the end of the day on September 14, 2017, or such later time and date to which the Offer is extended.

        Any shareholder who desires to tender Shares and whose certificates are not immediately available, or who cannot comply with the procedures for book-entry transfer described in this Offer to Purchase on a timely basis, or who cannot deliver all required documents to the Depositary prior to the expiration of the Offer, may nevertheless tender such Shares by following the procedures for guaranteed delivery set forth in Section 2 of the Offer to Purchase. See Instruction 2.

        A shareholder owning beneficially or of record an aggregate of fewer than 100 Shares and who tenders all such Shares and satisfies the other requirements set forth in Instruction 8, may have all such Shares purchased before proration, if any, of the purchase of other Shares pursuant to the Offer.

        Your attention is directed to the following:

1.

If you want to retain all your Shares, do not take any action.



2.

If you wish to maximize the chance that your Shares will be purchased by us, you should check the box in the section of this Letter of Transmittal captioned "Shares Tendered at a Price Determined Pursuant to the Offer". If you agree to accept the purchase price determined pursuant to the Offer, your Shares will be deemed to be tendered at the minimum price of $825 per share. You should understand that this election may lower the purchase price paid for all purchased Shares in the Offer and could result in the tendered Shares being purchased at the minimum price of $825 per Share, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.



3.

If you wish to select a specific price at which you will be tendering your Shares, you should select one of the boxes in the section captioned "Shares Tendered at a Price Determined by Shareholder" below and complete the other portions of this Letter of Transmittal as appropriate.

        We urge shareholders who hold Shares through a broker, dealer, commercial bank, trust company or other nominee to consult their nominee to determine whether transaction costs are applicable if they tender Shares through their nominee and not directly to the Depositary.

        QUESTIONS AND REQUESTS FOR ASSISTANCE OR FOR ADDITIONAL COPIES OF THE OFFER TO PURCHASE OR THIS LETTER OF TRANSMITTAL MAY BE DIRECTED TO THE INFORMATION AGENT OR THE DEALER MANAGERS AT THEIR RESPECTIVE ADDRESSES OR TELEPHONE NUMBERS SET FORTH AT THE END OF THIS LETTER OF TRANSMITTAL.

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PLEASE READ CAREFULLY THE INSTRUCTIONS SET FORTH BELOW BEFORE COMPLETING THIS LETTER OF TRANSMITTAL.

 

 

 

 

 

 

 

 

 

 

 

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PRICE PER SHARE AT WHICH SHARES ARE BEING TENDERED
(See Instruction 7)

CHECK ONLY THE BOX UNDER 1 OR 2 BELOW.

IF BOTH BOXES UNDER 1 OR 2 ARE CHECKED OR IF NEITHER OF THE BOXES UNDER 1 OR 2 ARE CHECKED, THERE IS NO VALID TENDER OF SHARES.

1.     SHARES TENDERED AT A PRICE DETERMINED PURSUANT TO THE OFFER

o

By checking this box INSTEAD OF ONE OF THE BOXES UNDER 2 BELOW, the undersigned hereby tenders Shares and is willing to accept the purchase price determined pursuant to the Offer. This action will maximize the chance of having the Company purchase all of the Shares tendered by the undersigned (subject to the possibility of proration). The undersigned understands that this election could result in the tendered shares being purchased at the minimum price of $825 per Share, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.

—OR—

2.     SHARES TENDERED AT A PRICE DETERMINED BY SHAREHOLDER

By checking ONE of the boxes below INSTEAD OF THE BOX UNDER 1 ABOVE, the undersigned hereby tenders Shares at the price checked the corresponding box in the section below captioned "Price (in Dollars) per Share at which Shares are Being Tendered". This action could result in none of the Shares being purchased if the purchase price for the Shares is less than the price checked. If the purchase price for the Shares is equal to or greater than the price checked, then the Shares purchased by the Company will be purchased at the purchase price. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. A shareholder who wishes to tender Shares at more than one price must complete a separate Letter of Transmittal for each price at which Shares are being tendered. The same Shares cannot be tendered (unless such Shares were previously withdrawn in accordance with the terms of the Offer) at more than one price.

PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING TENDERED.

CHECK ONLY ONE BOX BELOW. IF MORE THAN ONE BOX BELOW IS CHECKED OR IF NO BOX BELOW IS CHECKED, THERE IS NO VALID TENDER OF SHARES.

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ODD LOTS
(See Instruction 8)

        As described in Section 1 of the Offer to Purchase, under certain conditions, a shareholder owning beneficially or of record an aggregate of fewer than 100 Shares and who tenders all such Shares and satisfies the other requirements set forth in the Offer to Purchase, may have his or her Shares accepted for payment before any proration of other tendered Shares. This preference is not available to partial tenders, or to beneficial or record holders of an aggregate of 100 or more Shares, even if these holders have separate accounts representing fewer than 100 Shares. Accordingly, this section is to be completed only if Shares are being tendered by or on behalf of a person owning, beneficially or of record, an aggregate of fewer than 100 Shares. The undersigned either (check one box):

o

owns, whether beneficially or of record, an aggregate of fewer than 100 Shares and is tendering all such Shares; or

o

is a broker, dealer, commercial bank, trust company, or other nominee that (a) is tendering for the beneficial owner(s), Shares with respect to which it is the record holder, and (b) believes, based upon representations made to it by the beneficial owner(s), that each such person is the beneficial owner of an aggregate of fewer than 100 Shares and is tendering all such Shares.

        In addition, the undersigned is tendering all such Shares either (check one box):

o

at the Purchase Price, as the same will be determined by the Company in accordance with the terms of the Offer (persons checking this box should not check any box in the section captioned "Price (in Dollars) per Share at which Shares are Being Tendered"); or

o

at the price per Share indicated above in the section captioned "Price (in Dollars) per Share at which Shares are Being Tendered."

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NOTE: SIGNATURES MUST BE PROVIDED BELOW
PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY

Ladies and Gentlemen:

        The undersigned hereby tenders to White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), the above-described Common Shares, par value $1.00 per share, of the Company (the "Shares"), at the price indicated in this Letter of Transmittal, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase dated August 17, 2017 (the "Offer to Purchase"), receipt of which is hereby acknowledged, and in this Letter of Transmittal (which together, as they may be amended or supplemented from time to time, constitute the "Offer").

        Subject to, and effective upon, acceptance for payment for the Shares tendered herewith in accordance with the terms of the Offer (including, if the Offer is extended or amended, the terms or conditions of any such extension or amendment), the undersigned hereby sells, assigns and transfers to or upon the order of the Company all right, title and interest in and to all the Shares tendered hereby, or orders the registration of such Shares delivered by book-entry transfer, that are purchased pursuant to the Offer and hereby irrevocably constitutes and appoints the Depositary for the Offer, Computershare Trust Company, N.A. (the "Depositary"), the true and lawful agent and attorney-in-fact of the undersigned with respect to such Shares, with full power of substitution (such power of attorney being deemed to be an irrevocable power coupled with an interest), to:

(a)

deliver certificates for such Shares, or transfer ownership of such Shares on the account books maintained by The Depository Trust Company ("DTC"), together, in any such case, with all accompanying evidence of transfer and authenticity, to or upon the order of the Company, upon receipt by the Depositary, as the undersigned's agent, of the Purchase Price (as defined below) with respect to such Shares;



(b)

present certificates for such Shares for cancellation and transfer of such Shares on the Company's books; and



(c)

receive all benefits and otherwise exercise all rights of beneficial ownership of such Shares, all in accordance with the terms of the Offer.

        The undersigned hereby represents and warrants that:

(a)

the undersigned has full power and authority to tender, sell, assign and transfer the Shares tendered and that, when the same are accepted for payment by the Company, the Company will acquire good, marketable and unencumbered title thereto, free and clear of all security interests, liens, restrictions, claims, charges, encumbrances and other obligations relating to the sale or transfer of the Shares;



(b)

the undersigned will, on request by the Depositary or the Company, execute and deliver any additional documents deemed by the Depositary or the Company to be necessary or desirable to complete the sale, assignment and transfer of the Shares tendered, all in accordance with the terms of the Offer; and



(c)

the undersigned understands that tendering Shares pursuant to any one of the procedures described in Section 2 in the Offer to Purchase and in the instructions hereto will constitute the undersigned's acceptance of the terms and conditions of the Offer, including the undersigned's representation and warranty that: (i) the undersigned has a "net long position" in Shares at least equal to the Shares tendered within the meaning of Rule 14e-4 under the Securities Exchange Act of 1934 (as amended, the "Exchange Act") and (ii) such tender of Shares complies with Rule 14e-4 under the Exchange Act.


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        The undersigned understands that the purchase price will be the lowest price per Share (in increments of $1.00) not less than $825 (such price per Share or such other price that may be set forth in an amendment to the Offer referred to as the "Purchase Price") selected by the Company that will allow the Company to purchase the number of Shares sought in the Offer or, if a lesser number is properly tendered, all Shares that are properly tendered and not properly withdrawn, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions of the Offer, including the proration provisions thereof, and that the Company will promptly return all other Shares, including Shares not purchased because of proration.

        The undersigned understands that tenders of Shares pursuant to any one of the procedures described in Section 2 of the Offer to Purchase and in the instructions hereto will constitute a binding agreement between the undersigned and the Company upon the terms and subject to the conditions of the Offer.

        The undersigned recognizes that, under certain circumstances set forth in the Offer to Purchase, the Company may terminate or amend the Offer or may not be required to accept for payment any of the Shares tendered herewith or may accept for payment, fewer than all the Shares tendered herewith in accordance with the priority and proration provisions described in Section 1 of the Offer to Purchase.

        All authority conferred or agreed to be conferred in this Letter of Transmittal shall survive the death or incapacity of the undersigned, and any obligation of the undersigned hereunder shall be binding upon the heirs, personal representatives, successors and assigns of the undersigned. Except as stated in the Offer, this tender is irrevocable.

        Unless otherwise indicated under "Special Payment Instructions", please issue the check for the aggregate Purchase Price and/or return or issue the certificate(s) evidencing any Shares not tendered or not accepted for payment in the name(s) of the registered holder(s) appearing under "Description of Shares Tendered". Similarly, unless otherwise indicated under "Special Delivery Instructions", please mail the check for the aggregate Purchase Price and/or the certificate(s) evidencing any Shares not tendered or not accepted for payment (and accompanying documents, as appropriate) to the address(es) of the registered holder(s) appearing under "Description of Shares Tendered". In the event that both the "Special Delivery Instructions" and the "Special Payment Instructions" are completed, please issue the check for the aggregate Purchase Price and/or issue or return the certificate(s) evidencing any Shares not tendered or accepted for payment in the name(s) of, and deliver said check and/or certificate(s) to, the person or persons so indicated. In the case of book-entry delivery of Shares, please credit the account maintained at DTC with any Shares not accepted for payment. The undersigned recognizes that the Company has no obligation pursuant to the "Special Payment Instructions" to transfer any Shares from the name(s) of the registered holder(s) thereof if the Company does not accept for payment any of the Shares so tendered.

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PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY

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IMPORTANT TAX INFORMATION

        Under U.S. Federal income tax law, a shareholder whose tendered Shares are accepted for payment is required by law to provide the Depositary with such shareholder's correct taxpayer identification number ("TIN") on the Substitute Form W-9 below. If the Depositary is not provided with the correct TIN, the Internal Revenue Service ("IRS") may subject the shareholder or other payee to a $250 penalty. In addition, payments that are made to such shareholder or other payee with respect to Shares purchased pursuant to the Offer may be subject to 28% backup withholding.

        Certain shareholders (including, among others, certain corporations and certain foreign individuals) are not subject to these backup withholding and reporting requirements and should indicate their status by writing "exempt" across the face of the Substitute Form W-9. In order for a foreign individual to qualify as an exempt recipient, the shareholder must submit a Form W-8BEN or W-8BEN-E or other applicable form, signed under penalties of perjury, attesting to that individual's exempt status. A Form W-8BEN or W-8BEN-E can be obtained from the Depositary. See the enclosed "Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9" for more instructions.

        If backup withholding applies, the Depositary is required to withhold 28% of any such payments to be made to the shareholder or other payee. Backup withholding is not an additional tax. Rather, the tax liability of persons subject to backup withholding will be reduced by the amount of tax withheld. If backup withholding results in an overpayment of taxes, a refund may be obtained from the IRS, provided that the required information is timely given to the IRS.

        The box in Part 3 of the Substitute Form W-9 may be checked if the tendering shareholder has not been issued a TIN and has applied for a TIN or intends to apply for a TIN in the near future. If the box in Part 3 is checked, the shareholder or other payee must also complete the Certificate of Awaiting Taxpayer Identification Number below in order to avoid backup withholding. Notwithstanding that the box in Part 3 is checked and the Certificate of Awaiting Taxpayer Identification Number is completed, the Depositary will withhold 28% on all payments made prior to the time a properly certified TIN is provided to the Depositary.

        The shareholder is required to give the Depositary the TIN (e.g., social security number or employer identification number) of the record owner of the Shares or of the last transferee appearing on the transfers attached to, or endorsed on, the certificates evidencing the Shares. If the Shares are registered in more than one name or are not registered in the name of the actual owner, consult the enclosed "Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9" for additional guidance on which number to report.

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YOU MUST COMPLETE THE FOLLOWING CERTIFICATE IF YOU CHECKED
THE BOX IN PART 3 OF SUBSTITUTE FORM W-9

NOTE:

FAILURE TO COMPLETE AND RETURN THIS SUBSTITUTE FORM W-9 MAY RESULT IN BACKUP WITHHOLDING OF 28% OF ANY PAYMENTS MADE TO YOU PURSUANT TO THE OFFER. PLEASE REVIEW THE ENCLOSED GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9 FOR ADDITIONAL INFORMATION.

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GUIDELINES FOR CERTIFICATION OF TAXPAYER
IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9

        Guidelines for Determining the Proper Identification Number to Give the Payer—Social Security numbers have nine digits separated by two hyphens: i.e., 000-00-0000. Employer identification numbers have nine digits separated by only one hyphen: i.e., 00-0000000. The table below will help determine the number to give the payer.

 

1.

List first and circle the name of the person whose number you furnish. If only one person on a joint account has an SSN, that person's number must be furnished.

2.

Circle the minor's name and furnish the minor's SSN.

3.

You must show your individual name and you may also enter your business or "doing business as" name on the "Business name/disregarded entity" name line. You may use either your SSN or EIN (if you have one), but the IRS encourages you to use your SSN.

4.

List first and circle the name of the legal trust, estate, or pension trust. (Do not furnish the TIN of the personal representative or trustee unless the legal entity itself is not designated in the account title.)

*

Note. Grantor must also provide a Form W-9 to trustee of trust.

        Note:    If no name is circled when more than one name is listed, the number will be considered to be that of the first name listed.

   

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        Resident Alien Individuals:    If you are a resident alien individual and you do not have and are not eligible to get an SSN, your TIN is your IRS individual taxpayer identification number (ITIN). Enter it in the social security number box. If you do not have an ITIN, see How to Obtain a TIN below.

How to Obtain a TIN

        If you don't have a taxpayer identification number or you don't know your number, obtain Form SS-5, Application for a Social Security Number Card, or Form SS-4, Application for Employer Identification Number, at the local office of the Social Security Administration or the Internal Revenue Service ("IRS") and apply for a number. Resident alien individuals who are not eligible to get an SSN and need an ITIN should obtain Form W-7, Application for Individual Taxpayer Identification Number, from the IRS. You may obtain Form SS-4 and Form W-7 from the IRS's website at http://www.irs.gov.

Payees Exempt from Backup Withholding

        Payees exempt from backup withholding on all payments include the following:

•

An organization exempt from tax under section 501(a), any IRA, or a custodial account under section 403(b)(7) if the account satisfies the requirements of section 401(f)(2).



•

The United States or any of its agencies or instrumentalities.



•

A state, the District of Columbia, a possession of the United States or any of their political subdivisions or instrumentalities.



•

A foreign government or any of its political subdivisions, agencies or instrumentalities.



•

An international organization or any of its agencies or instrumentalities.

        Other payees that may be exempt from backup withholding include:

•

A corporation.



•

A foreign central bank of issue.



•

A dealer in securities or commodities required to register in the United States, the District of Columbia or a possession of the United States.



•

A futures commission merchant registered with the Commodity Futures Trading Commission.



•

A real estate investment trust.



•

An entity registered at all times during the tax year under the Investment Company Act of 1940.



•

A common trust fund operated by a bank under section 584(a).



•

A financial institution.



•

A middleman known in the investment community as a nominee or custodian.



•

A trust exempt from tax under section 664 or described in section 4947.

        Payments of dividends and patronage dividends not generally subject to backup withholding include the following:

•

Payments to nonresident aliens subject to withholding under section 1441.



•

Payments to partnerships not engaged in a trade or business in the United States and that have at least one nonresident alien partner.



•

Payments of patronage dividends where the amount received is not paid in money.



•

Payments made by certain foreign organizations.


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        Payments of interest not generally subject to backup withholding include the following:

•

Payments of interest on obligations issued by individuals. Note: You may be subject to backup withholding if this interest is $600 or more and is paid in the course of the payer's trade of business and you have not provided your correct taxpayer identification number to the payer.



•

Payments described in section 6049(b)(5) to nonresident aliens.



•

Payments on tax-free covenant bonds under section 1451.



•

Payments made by certain foreign organizations.

        Exempt payees described above should file Substitute Form W-9 to avoid possible erroneous backup withholding. FURNISH YOUR TAXPAYER IDENTIFICATION NUMBER, WRITE "EXEMPT" ON THE FACE OF THE FORM IN PART II, SIGN AND DATE THE FORM, AND RETURN IT TO THE PAYER.

        Certain payments, other than interest, dividends and patronage dividends that are not subject to information reporting are also not subject to backup withholding. For details, see the regulations under sections 6041, 6041A(a), 6045 and 6050A.

        Privacy Act Notice.—Section 6109 requires most recipients of dividend, interest or other payments to give their correct taxpayer identification numbers to payers who must report the payments to the IRS. The IRS uses the numbers for identification purposes and to help verify the accuracy of tax returns. Payers must be given the numbers whether or not recipients are required to file tax returns. Payers must generally withhold 28% (or such other rate specified by the Internal Revenue Code) of taxable interest, dividend and certain other payments to a payee who does not furnish a taxpayer identification number to a payer. Certain penalties may also apply.

Penalties

1.

Penalty for Failure to Furnish Taxpayer Identification Number.—If you fail to furnish your correct taxpayer identification number to a payer, you are subject to a penalty of $250 for each such failure unless your failure is due to reasonable cause and not to willful neglect.

2.

Civil Penalty for False Information With Respect to Withholding.—If you make a false statement with no reasonable basis which results in no imposition of backup withholding, you are subject to a penalty of $500.

3.

Criminal Penalty for Falsifying Information.—Willfully falsifying certifications or affirmations may subject you to criminal penalties including fines and/or imprisonment.

        FOR ADDITIONAL INFORMATION CONTACT YOUR TAX ADVISOR OR THE INTERNAL REVENUE SERVICE.

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INSTRUCTIONS
Forming Part of the Terms and Conditions of the Offer

        1.    Guarantee of Signatures.    Signatures on this Letter of Transmittal must be guaranteed by a firm which is a member of the Securities Transfer Agent Medallion Program, the New York Stock Exchange Medallion Signature Program or the Stock Exchange Medallion Program (each, an "Eligible Institution"), except in cases where Shares are tendered (i) by a registered holder (which term, for purposes of this Letter of Transmittal, will include any participant in DTC whose name appears on a security position listing as the owner of the Shares) of Shares who has not completed either the box entitled "Special Payment Instructions" or the box entitled "Special Delivery Instructions" on this Letter of Transmittal or (ii) for the account of an Eligible Institution. Shareholders may also need to have any certificates they deliver endorsed or accompanied by a stock power, and the signatures on these documents also may need to be guaranteed. See Instruction 5.

        2.    Delivery of Letter of Transmittal and Shares; Guaranteed Delivery Procedures.    This Letter of Transmittal is to be used only if (a) certificates are to be forwarded with it to the Depositary or (b) delivery of Shares is to be made by book-entry transfer pursuant to the procedure set forth in Section 2 of the Offer to Purchase. Certificates for all physically delivered Shares, or a confirmation of a book-entry transfer of all Shares delivered electronically into the Depositary's account at DTC, together in each case with a properly completed and duly executed Letter of Transmittal, with any required signature guarantees, and any other documents required by this Letter of Transmittal, must be received by the Depositary at one of its addresses set forth on the front page of this Letter of Transmittal before the Expiration Time (as defined in the Offer to Purchase). Delivery of documents to DTC does not constitute delivery to the Depositary.

        Shareholders whose certificates are not immediately available (or who cannot follow the procedures for book-entry transfer on a timely basis) or who cannot transmit this Letter of Transmittal and all other required documents to reach the Depositary before the Expiration Time, may nevertheless tender their Shares pursuant to the guaranteed delivery procedure set forth in Section 2 of the Offer to Purchase. Pursuant to such procedure: (a) such tender must be made by or through an Eligible Institution, (b) the Depositary must receive by mail, express or overnight delivery, or by email before the Expiration Time, a properly completed and duly executed Notice of Guaranteed Delivery substantially in the form the Company has provided with the Offer to Purchase and (c) the certificates for all tendered Shares in proper form for transfer (or confirmation of a book-entry transfer of all such Shares into the Depositary's account at DTC), together with a properly completed and duly executed Letter of Transmittal and any other documents required by this Letter of Transmittal, must be received by the Depositary within two business days after the date of execution of such Notice of Guaranteed Delivery, all as provided in Section 2 of the Offer to Purchase.

        The method of delivery of all documents, including share certificates, this Letter of Transmittal and any other required documents, is at the option and risk of the tendering shareholder and delivery will be deemed made only when actually received by the Depositary (including, in the case of a book-entry transfer, by book-entry confirmation). If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all cases, sufficient time should be allowed to ensure timely delivery to the Depositary prior to the Expiration Time.

        No alternative, conditional or contingent tenders will be accepted, and no fractional Shares will be purchased. By executing this Letter of Transmittal, each tendering shareholder waives any right to receive any notice of the acceptance of such shareholder's tender.

        3.    Inadequate Space.    If the space provided in the box entitled "Description of Shares Tendered" is inadequate, the certificate numbers and/or the number of Shares should be listed on a separate signed schedule and attached to this Letter of Transmittal.

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        4.    Partial Tenders and Unpurchased Shares.    (Not applicable to shareholders who deliver Shares by book-entry transfer). If fewer than all the Shares evidenced by any certificate delivered to the Depositary are to be tendered, fill in the number of Shares that are to be tendered in the box entitled "Number of Shares Tendered". If such Shares are purchased, a new certificate for the remainder of the Shares evidenced by the old certificate(s) will be sent to and in the name of the registered holder(s) (unless otherwise specified by such holder(s) having completed either of the boxes entitled "Special Delivery Instructions" or "Special Payment Instructions" on this Letter of Transmittal) promptly following the expiration or termination of the Offer. All Shares represented by the certificate(s) listed and delivered to the Depositary will be deemed to have been tendered unless otherwise indicated.

        5.    Signatures on Letter of Transmittal; Share Powers; and Endorsements.    

(a)

If this Letter of Transmittal is signed by the registered holder(s) of the Shares tendered herewith, the signature(s) must correspond exactly with the name(s) as written on the face of the certificates without any change whatsoever.

(b)

If any of the Shares tendered herewith are registered in the names of two or more joint owners, each such owner must sign this Letter of Transmittal.

(c)

If any of the Shares tendered herewith are registered in different names on different certificates, it will be necessary to complete, sign and submit as many separate Letters of Transmittal as there are different registrations of certificates.

(d)

If this Letter of Transmittal is signed by the registered holder(s) of the Shares tendered herewith, no endorsements of certificates or separate share powers are required unless payment is to be made and/or certificates for Shares not tendered or not purchased are to be issued to a person other than the registered holder(s). If this Letter of Transmittal is signed by a person other than the registered holder(s) of the Shares tendered herewith, however, the certificates must be endorsed or accompanied by appropriate share powers, in either case, signed exactly as the name(s) of the registered holder(s) appear on the certificates for such Shares. Signatures on any such certificates or share powers must be guaranteed by an Eligible Institution. See Instruction 1.

(e)

If this Letter of Transmittal or any share powers are signed, or any certificate is endorsed, by a trustee, executor, administrator, guardian, attorney-in-fact, agent, officer of a corporation or other person acting in a fiduciary or representative capacity, such person should so indicate when signing and proper evidence satisfactory to the Company of the authority of such person so to act must be submitted.

        6.    Share Transfer Taxes.    The Company will pay any share transfer taxes with respect to the transfer and sale of Shares to it or its order pursuant to the Offer. If, however, payment of the aggregate Purchase Price is to be made to, or certificates for Shares not tendered or accepted for purchase are to be registered in the name of, any person other than the registered holder, or if tendered certificates are registered in the name of any person other than the person(s) signing this Letter of Transmittal, the amount of any share transfer taxes (whether imposed on the registered holder or such person) payable on account of the transfer to such person will be deducted from the aggregate Purchase Price unless satisfactory evidence of payment of such taxes or exemption therefrom is submitted.

        7.    Indication of Price at which Shares are Being Tendered.    If you want to tender your Shares you must properly complete the pricing section of this Letter of Transmittal, which is called "Price per Share at which Shares are Being Tendered". You must check either the box under "1. Shares Tendered at a Price Determined Pursuant to the Offer" or the box under "2. Shares Tendered at a Price Determined by the Shareholder". If both of such boxes are checked, or if neither of such boxes is checked, there is no valid tender of Shares. If you check the box under "2. Shares Tendered at a Price Determined by the Shareholder", you must additionally check one box in the section captioned "Price

VOLUNTARY CORPORATE ACTIONS COY: WTM

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(in Dollars) per Share at which Shares are Being Tendered." corresponding to the price at which you want to tender your Shares; if more than one of such boxes is checked or if none of such boxes is checked, there is no valid tender of Shares. If you want to tender portions of your Shares at more than one price, you must complete a separate Letter of Transmittal for each price at which you tender Shares. However, the same Shares cannot be tendered at more than one price, unless such Shares were previously withdrawn as provided in Section 3 of the Offer to Purchase. By checking the box under "1. Shares Tendered at a Price Determined Pursuant to the Offer" instead of the box under "2. Shares Tendered at a Price Determined by the Shareholder", you are tendering Shares and are willing to accept the Purchase Price selected by the Company in accordance with the terms of the Offer. This action will maximize the chance of having the Company purchase your Shares (subject to the possibility of proration). You should understand that checking box under "1. Shares Tendered at a Price Determined Pursuant to the Offer" may lower the Purchase Price paid for all purchased Shares in the Offer and could result in the tendered Shares being purchased at the minimum price of $825, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.

        8.    Odd Lots.    As described in Sections 1 and 2 of the Offer to Purchase, if the number of Shares properly tendered at or below the Purchase Price and not properly withdrawn before the Expiration Time is greater than 500,000 (or such greater number of Shares as the Company may elect to purchase pursuant to the Offer, subject to applicable law), the Company, upon the terms and subject to the conditions of the Offer, will accept Shares for purchase first from all Shares properly tendered at or below the Purchase Price and not properly withdrawn before the Expiration Time by any shareholder who owns beneficially or of record an aggregate of fewer than 100 Shares and who tendered all Shares beneficially owned by such person (partial tenders of Shares will not qualify for this preference) and completed the box captioned "Odd Lots" in this Letter of Transmittal and, if applicable, on the Notice of Guaranteed Delivery. This preference will not be available unless the box above entitled "Odd Lots" is completed.

        9.    Irregularities.    All questions as to the price to be paid for the Shares to be accepted and the validity, form, eligibility (including time of receipt) and acceptance for payment of any tender of Shares will be determined by the Company in its reasonable discretion, which determination shall be final and binding on all parties. The Company reserves the absolute right to reject any and all tenders reasonably determined by it not to be in proper form or the acceptance for payment of which may, in the opinion of its counsel, be unlawful. The Company also reserves the absolute right to waive any condition of the Offer (except as provided in Section 8 of the Offer to Purchase) or any defect or irregularity in the tender of any particular Shares or any particular shareholder, whether or not similar defects or irregularities are waived in the case of other shareholders, and the Company's reasonable interpretation of the terms and conditions of the Offer (including these instructions) will be final and binding on all persons. No tender of Shares will be deemed to have been properly made until all defects and irregularities have been cured or waived to the satisfaction of the Company. The Company will not be liable for failure to waive any condition of the Offer, or any defect or irregularity in any tender of Shares. Neither the Company, nor any other person will be under any duty to give notification of any defects or irregularities in tenders or incur any liability for failure to give any such notification. The Company's reasonable interpretation of the terms of and conditions to the Offer, including this Letter of Transmittal and the instructions hereto, will be final and binding on all persons participating in the Offer. Any determination by the Company as to the validity, form, eligibility and acceptance of Shares for payment, or any interpretation by the Company as to the terms and conditions of the Offer, is subject to applicable law and, if challenged by shareholders in a lawsuit, to the judgment of a court of competent jurisdiction.

        10.    Special Payment and Delivery Instructions.    If the check for the aggregate Purchase Price of any Shares purchased is to be issued to, or any Shares not tendered or not purchased are to be returned in the name of, a person other than the person(s) signing this Letter of Transmittal or if the

VOLUNTARY CORPORATE ACTIONS COY: WTM

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check or any certificates for Shares not tendered or not purchased are to be mailed to someone other than the person(s) signing this Letter of Transmittal or to the person(s) signing this Letter of Transmittal at an address other than that shown in the box entitled "Descriptions of Shares Tendered", the boxes entitled "Special Payment Instructions" and/or "Special Delivery Instructions" on this Letter of Transmittal should be completed.

        11.    Request for Assistance or Additional Copies.    Requests for assistance or additional copies of the Offer to Purchase, this Letter of Transmittal or the Notice of Guaranteed Delivery may be directed to the Information Agent at its address, telephone numbers or email address set forth below.

        12.    Substitute Form W-9.    Except as provided above under "Important Tax Information", each tendering shareholder is required to provide the Depositary with a correct TIN on Substitute Form W-9 which is provided under "Important Tax Information" above. Failure to provide the information on the form may subject the tendering shareholder to a $250 penalty and a 28% Federal backup withholding tax may be imposed on the payments made to the shareholder or other payee with respect to Shares purchased pursuant to the Offer.

        13.    Non-U.S. Shareholder Withholding.    Non-U.S. shareholders should note that the 30% U.S. withholding tax generally applicable to distributions by U.S. corporations should not apply to the proceeds payable pursuant to the Offer (however, as indicated above under "Important Tax Information", U.S. Federal backup withholding tax may be applicable). Non-U.S. shareholders should not use Substitute Form W-9. Instead, non-U.S. shareholders must provide an appropriate form W-8 or suitable substitute.

        14.    Lost, Destroyed or Stolen Certificates.    If any certificate(s) for part or all of your shares has been lost, stolen, destroyed or mutilated, you should contact Computershare Trust Company, N.A., as Depositary for the Company, at (877) 373-6374 (toll free in the United States and Canada) or (781) 575-2879 (outside the United States and Canada) to arrange for replacement of lost securities. This Letter of Transmittal and related documents cannot be processed until the procedures for replacing lost, stolen, destroyed or mutilated certificates have been followed and the tender is deemed by the Depositary to be in proper form prior to the Expiration Time. You are urged to contact the Depositary immediately if you wish to tender Shares that you are unable to locate or have been destroyed. If you do not contact the Depositary promptly upon receipt of this Letter of Transmittal, you may not receive instructions for replacement in time to properly surrender your Shares for tender by the Expiration Time.

        The Letter of Transmittal, certificates for Shares and any other required documents should be sent or delivered by each shareholder of the Company or such shareholder's broker, dealer, commercial bank, trust company or other nominee to the Depositary at one of its addresses set forth below.

VOLUNTARY CORPORATE ACTIONS COY: WTM

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The Depositary for the Offer is:

Computershare Trust Company, N.A.

        DELIVERY OF THE LETTER OF TRANSMITTAL TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE WILL NOT CONSTITUTE A VALID BINDING DELIVERY TO THE DEPOSITARY.

        Any questions or requests for assistance or for additional copies of this Offer to Purchase, the Letter of Transmittal or the Notice of Guaranteed Delivery may be directed to the Information Agent. Shareholders may also contact their broker, dealer, commercial bank, trust company or other nominee for assistance concerning the Offer.

The Information Agent for the Offer is:
D.F. King & Co., Inc.
48 Wall Street, 22nd Floor
New York, NY 10005

Banks and Brokerage Firms Please Call Collect: (212) 269-5550

All Others Call Toll Free (800) 893-5865

Email: wtm@dfking.com

The Dealer Managers for the Offer are:

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Exhibit (a)(1)(B)

LETTER OF TRANSMITTAL To Tender Common Shares of WHITE MOUNTAINS INSURANCE GROUP, LTD. Pursuant to the Offer to Purchase Dated August 17, 2017
The Depositary for the Offer is: Computershare Trust Company, N.A.
PRICE PER SHARE AT WHICH SHARES ARE BEING TENDERED (See Instruction 7)
CHECK ONLY THE BOX UNDER 1 OR 2 BELOW.
ODD LOTS (See Instruction 8)
NOTE: SIGNATURES MUST BE PROVIDED BELOW PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY
PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY
IMPORTANT TAX INFORMATION
GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9
INSTRUCTIONS Forming Part of the Terms and Conditions of the Offer

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Exhibit (a)(1)(C)

        NOTICE OF GUARANTEED DELIVERY
(Not to be used for Signature Guarantee)

WHITE MOUNTAINS INSURANCE GROUP, LTD.

        As set forth in Section 2 of the Offer to Purchase (as defined below), this form must be used to accept the Offer (as defined below) if (1) certificates evidencing your tendered Common Shares of White Mountains Insurance Group, Ltd. (the "Company"), par value of $1.00 per share (the "Shares"), are not immediately available or cannot be delivered to the Depositary before the Expiration Time (as defined in the Offer to Purchase), (2) the procedures for book-entry transfer described in Section 2 of the Offer to Purchase cannot be completed before the Expiration Time or (3) time will not permit all required documents to reach the Depositary before the Expiration Time. This form, signed and properly completed, may be delivered to Computershare Trust Company, N.A. (the "Depositary") by mail or overnight delivery or transmitted by email to the Depositary. See Section 2 of the Offer to Purchase.

The Depositary for the Offer is:

Computershare Trust Company, N.A.

By email: canoticeofguarantee@computershare.com

        DELIVERY OF THIS NOTICE OF GUARANTEED DELIVERY TO AN ADDRESS, OR TRANSMISSION OF INSTRUCTIONS VIA EMAIL OTHER THAN AS SET FORTH ABOVE, WILL NOT CONSTITUTE A VALID DELIVERY. DELIVERIES TO THE COMPANY, THE INFORMATION AGENT, THE DEALER MANAGERS, OR THE DEPOSITORY TRUST COMPANY ("DTC") WILL NOT BE FORWARDED TO THE DEPOSITARY AND WILL NOT CONSTITUTE VALID DELIVERY.

        THIS NOTICE OF GUARANTEED DELIVERY IS NOT TO BE USED TO GUARANTEE SIGNATURES. IF A SIGNATURE ON A LETTER OF TRANSMITTAL IS REQUIRED TO BE GUARANTEED BY AN ELIGIBLE INSTITUTION (AS DEFINED IN THE OFFER TO PURCHASE) UNDER THE INSTRUCTIONS TO THE LETTER OF TRANSMITTAL, SUCH SIGNATURE GUARANTEE MUST APPEAR IN THE APPLICABLE SPACE PROVIDED IN THE SIGNATURE BOX ON THE LETTER OF TRANSMITTAL.

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Ladies and Gentlemen:

        The undersigned hereby tenders to the Company, upon the terms and subject to the conditions set forth in its Offer to Purchase dated August 17, 2017 (the "Offer to Purchase"), and in the related Letter of Transmittal (which together, as they may be amended or supplemented from time to time, constitute the "Offer"), receipt of which is hereby acknowledged, the number (indicated herein) of the Shares, listed below at the price indicated below, pursuant to the guaranteed delivery procedures set forth in Section 2 of the Offer to Purchase.

NUMBER OF SHARES TO BE TENDERED:              SHARES
Certificate Nos. (if available):

PRICE PER SHARE AT WHICH SHARES ARE BEING TENDERED
(See Instruction 7 of the Letter of Transmittal)

CHECK ONLY THE BOX UNDER 1 OR 2 BELOW. IF BOTH BOXES UNDER 1 OR 2 ARE CHECKED, OR IF NEITHER OF THE BOXES UNDER 1 OR 2 ARE CHECKED, THERE IS NO VALID TENDER OF SHARES.

1.     SHARES TENDERED AT A PRICE DETERMINED PURSUANT TO THE OFFER

o

By checking this box INSTEAD OF ONE OF THE BOXES UNDER 2 BELOW, the undersigned hereby tenders Shares and is willing to accept the purchase price determined pursuant to the Offer. This action will maximize the chance of having the Company purchase all of the Shares tendered by the undersigned (subject to the possibility of proration). You should understand that this election may lower the purchase price paid for all purchased Shares in the Offer and could result in the tendered Shares being purchased at the minimum price of $825 per Share, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.

—OR—

2.     SHARES TENDERED AT A PRICE DETERMINED BY SHAREHOLDER

By checking ONE of the boxes below INSTEAD OF THE BOX UNDER 1 ABOVE, the undersigned hereby tenders Shares at the price checked the corresponding box in the section below captioned "Price (in Dollars) per Share at which Shares are Being Tendered". This action could result in none of the Shares being purchased if the purchase price for the Shares is less than the price checked. If the purchase price for the Shares is equal to or greater than the price checked, then the Shares purchased by the Company will be purchased at the purchase price. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. A shareholder who wishes to tender Shares at more than one price must complete a separate Letter of Transmittal for each price at which Shares are being tendered. The same Shares cannot be tendered (unless previously properly withdrawn in accordance with the terms of the Offer) at more than one price.

PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING TENDERED.

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CHECK ONLY ONE BOX BELOW. IF MORE THAN ONE BOX BELOW IS CHECKED OR IF NO BOX BELOW IS CHECKED, THERE IS NO VALID TENDER OF SHARES.

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ODD LOTS
(See Instruction 8 of the Letter of Transmittal)

        As described in Section 1 of the Offer to Purchase, under certain conditions, a shareholder owning beneficially or of record an aggregate of fewer than 100 Shares and who tenders all such Shares and satisfies the other requirements set forth in the Offer to Purchase, may have his or her Shares accepted for payment before any proration of other tendered Shares. This preference is not available to partial tenders, or to beneficial or record holders of an aggregate of 100 or more Shares, even if these holders have separate accounts representing fewer than 100 Shares. Accordingly, this section is to be completed only if Shares are being tendered by or on behalf of a person owning, beneficially or of record, an aggregate of fewer than 100 Shares. The undersigned either (check one box):

o

owns, whether beneficially or of record, an aggregate of fewer than 100 Shares and is tendering all such Shares; or

o

is a broker, dealer, commercial bank, trust company, or other nominee that (a) is tendering for the beneficial owner(s), Shares with respect to which it is the record holder, and (b) believes, based upon representations made to it by the beneficial owner(s), that each such person is the beneficial owner of an aggregate of fewer than 100 Shares and is tendering all such Shares.

        In addition, the undersigned is tendering all such Shares either (check one box):

o

at the Purchase Price, as the same will be determined by the Company in accordance with the terms of the Offer (persons checking this box should not check any box in the section captioned "Price (in Dollars) per Share at which Shares are Being Tendered"); or

o

at the price per Share indicated above in the section captioned "Price (in Dollars) per Share at which Shares are Being Tendered".

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GUARANTEE
(Not to be used for signature guarantee)

        The undersigned, a financial institution that is a participant in the Securities Transfer Agent Medallion Program, the New York Stock Exchange Medallion Signature Program or the Stock Exchange Medallion Program, hereby guarantees (i) that the above-named person(s) has a net long position in the Shares being tendered within the meaning of Rule 14e-4 promulgated under the Securities Exchange Act of 1934, as amended, (ii) that such tender of Shares complies with Rule 14e-4 and (iii) to deliver to the Depositary at one of its addresses set forth above certificate(s) for the Shares tendered hereby, in proper form for transfer, or a confirmation of the book-entry transfer of the Shares into the Depositary's account at The Depository Trust Company, together with a properly completed and duly executed Letter of Transmittal, with any required signature guarantees, or an Agent's Message (as defined in the Offer to Purchase) in the case of a book-entry transfer, and any other required documents, within two business days after the date of receipt by the Depositary.

        The eligible institution that completes this form must communicate the guarantee to the Depositary and must deliver the Letter of Transmittal or an Agent's Message (as defined in the Offer to Purchase) and certificates for shares to the Depositary or complete the procedures for book-entry transfer deliveries within the time period set forth herein. Failure to do so could result in financial loss to such eligible institution.

        THIS FORM IS NOT TO BE USED TO GUARANTEE SIGNATURES. IF A SIGNATURE ON A LETTER OF TRANSMITTAL IS REQUIRED TO BE GUARANTEED BY AN ELIGIBLE INSTITUTION UNDER THE INSTRUCTIONS THERETO, SUCH SIGNATURE GUARANTEE MUST APPEAR IN THE APPLICABLE SPACE PROVIDED IN THE SIGNATURE BOX ON THE LETTER OF TRANSMITTAL.

        DO NOT SEND SHARE CERTIFICATES WITH THIS FORM. YOUR SHARE CERTIFICATES MUST BE SENT WITH THE LETTER OF TRANSMITTAL.

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Exhibit (a)(1)(C)

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Exhibit (a)(1)(D)

        OFFER TO PURCHASE FOR CASH
by
WHITE MOUNTAINS INSURANCE GROUP, LTD.
of
UP TO 500,000 OF ITS COMMON SHARES
at a Purchase Price not greater than $875
or less than $825 per Share

August 17, 2017

To Brokers, Dealers, Commercial Banks, Trust Companies and Other Nominees:

        We have been appointed by White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), to act as Dealer Managers in connection with its offer to purchase 500,000 of its outstanding common shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase dated August 17, 2017 (the "Offer to Purchase"), and in the related Letter of Transmittal (the "Letter of Transmittal" and, together with the Offer to Purchase, as they may be amended or supplemented from time to time, the "Offer"). The Company may elect, but is not obligated, to purchase additional Shares pursuant to the Offer. The Offer is not conditioned upon any minimum number of Shares being properly tendered but is subject to certain other conditions.

        The Company will determine a single per Share price, not greater than $875 or less than $825 per Share, that it will pay for the Shares properly tendered and not properly withdrawn pursuant to the Offer, taking into account the number of Shares so tendered and the prices specified by tendering shareholders. The Company will select the lowest purchase price (in increments of $1.00) that will allow it to purchase 500,000 Shares (or such lesser number as are properly tendered and not properly withdrawn) pursuant to the Offer. The Company will purchase all Shares properly tendered at prices at or below the purchase price and not properly withdrawn upon the terms and subject to the conditions set forth in the Offer to Purchase and the related Letter of Transmittal, including the provisions relating to "odd lot" tenders and proration described in the Offer to Purchase. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. In the event that more than 500,000 Shares are tendered in the Offer, the Company may exercise its right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of its outstanding Shares without extending the Expiration Time. The Company also expressly reserves the right, in its sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law. See Sections 1 and 14 of the Offer to Purchase. Shares tendered at prices in excess of the Purchase Price and Shares not purchased because of proration will be returned at the Company's expense to the shareholders who tendered such Shares promptly after the Expiration Time.

        Please contact your clients for whom you hold Shares registered in your name (or in the name of your nominee) or who hold Shares registered in their own names. Please bring the Offer to their attention as promptly as possible. No fees or commissions (other than fees paid by the Company to the Information Agent, the Depositary and the Dealer Managers as described in the Offer to Purchase) will be payable to brokers, dealers or other persons for soliciting tenders of Shares pursuant to the Offer. The Company will, however, upon request, reimburse you for customary mailing and handling expenses incurred by you in forwarding any of the enclosed materials to your clients. No shareholders

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will be required to pay transfer taxes on the transfer to the Company of Shares purchased pursuant to the Offer, subject to Instruction 6 of the Letter of Transmittal.

        For your information and for forwarding to your clients, we are enclosing the following documents:

(1)

Offer to Purchase;



(2)

Letter of Transmittal to be used by holders of Shares to tender Shares and for the information of your clients;



(3)

Letter from the Chief Executive Officer of the Company, G. Manning Rountree, to shareholders;



(4)

Form of Notice of Guaranteed Delivery to be used to accept the Offer and tender Shares pursuant to the Offer if the procedure for book-entry transfer cannot be completed by the Expiration Time or if all other required documents cannot be delivered to the Depositary by the Expiration Time;



(5)

Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9;



(6)

Letter that may be sent to your clients for whose accounts you hold Shares registered in your name (or in the name of your nominee), with space provided for obtaining such clients' instructions with regard to the Offer; and



(7)

Return envelope addressed to Computershare Trust Company, N.A., the Depositary.

        Shareholders must make their own decision as to whether to tender their Shares and, if so, how many Shares to tender and at what price or prices. Your clients should read carefully the information set forth or incorporated by reference in the Offer to Purchase and in the related Letter of Transmittal, including the Company's purpose for making the Offer.

        Certain conditions to the Offer are described in Section 8 of the Offer to Purchase. To be valid, all tenders must be in proper form as described in Section 2 of the Offer to Purchase.

        We urge you to contact your clients promptly. Please note that the Offer, the proration period and withdrawal rights will expire at 12:00 midnight, New York City time, at the end of the day on September 14, 2017, unless extended.

        Under no circumstances will interest be paid on the purchase price of the Shares regardless of any extension of, or amendment to, the Offer or any delay in paying for such Shares.

        None of the Company, our Board of Directors, the Dealer Managers, the Depositary or the Information Agent makes any recommendation as to whether any shareholder should tender Shares pursuant to the Offer or as to the purchase price or prices at which a shareholder may choose to tender them. Each of your clients must make their own decision after consulting with his or her own advisors whether to tender their Shares and, if so, how many Shares to tender and the price or prices at which they will tender them. In doing so, your clients should read carefully the information in the Offer, including the purposes and effects of the Offer.

        Additional copies of the enclosed material may be obtained from the undersigned. Any questions you may have with respect to the Offer should be directed to D.F. King & Co., Inc. at (212) 269-5550 or by email at wtm@dfking.com.

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        NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL RENDER YOU OR ANY OTHER PERSON THE AGENT OF THE COMPANY, THE DEPOSITARY, THE DEALER MANAGERS, THE INFORMATION AGENT OR ANY AFFILIATE OF THEM OR AUTHORIZE YOU OR ANY OTHER PERSON TO MAKE ANY STATEMENTS OR USE ANY MATERIAL ON BEHALF OF ANY OF THEM WITH RESPECT TO THE OFFER, OTHER THAN THE MATERIAL ENCLOSED HEREWITH AND THE STATEMENTS SPECIFICALLY CONTAINED IN SUCH MATERIAL.

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Exhibit (a)(1)(D)

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Exhibit (a)(1)(E)

OFFER TO PURCHASE FOR CASH
by
WHITE MOUNTAINS INSURANCE GROUP, LTD.
of
UP TO 500,000 OF ITS COMMON SHARES
at a Purchase Price not greater than $875
or less than $825 per Share

August 17, 2017

To Our Clients:

        Enclosed for your consideration is the Offer to Purchase dated August 17, 2017, (the "Offer to Purchase") of White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), and a related specimen Letter of Transmittal (the "Letter of Transmittal" and, together with the Offer to Purchase, as they may be amended or supplemented from time to time, the "Offer"), pursuant to which the Company is offering to purchase 500,000 of its outstanding common shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer. The Company may elect, but shall not be obligated, to purchase additional Shares pursuant to the Offer. The Offer to Purchase and a specimen Letter of Transmittal are being forwarded to you as the beneficial owner of Shares held by us in your account but not registered in your name. A tender of such Shares can be made only by us as the holder of record and only pursuant to your instructions. All capitalized terms used and not defined herein shall have the meanings ascribed to such terms in the Offer to Purchase.

        The Company will determine a single per Share price, not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, that it will pay for the Shares properly tendered and not properly withdrawn pursuant to the Offer, taking into account the number of Shares so tendered and the prices specified by tendering shareholders. The Company will select the lowest price not less than $825 (in increments of $1.00) (such price per Share referred to as the "Purchase Price") within the price range specified above that will allow it to purchase 500,000 Shares (or such lesser number as are properly tendered and not properly withdrawn) pursuant to the Offer. The Company will purchase all Shares properly tendered at prices at or below the Purchase Price and not withdrawn upon the terms and subject to the conditions set forth in the Offer to Purchase and the related Letter of Transmittal, including the provisions relating to "odd lot" tenders and proration described in the Offer to Purchase. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. In the event that more than 500,000 Shares are tendered in the Offer, the Company may exercise its right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of its outstanding Shares without extending the Expiration Time. The Company also expressly reserves the right, in its sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law. See Sections 1 and 14 of the Offer to Purchase. Shares tendered at prices in excess of the Purchase Price and Shares not purchased because of proration will be returned at the Company's expense to the shareholders who tendered such Shares promptly after the Expiration Time.

        If the number of Shares properly tendered and not properly withdrawn before the Expiration Time is less than or equal to 500,000 Shares (or such greater number of Shares as the Company may elect to purchase pursuant to the Offer, subject to applicable law), the Company will, on the terms and subject

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to the conditions of the Offer, purchase at the purchase price selected by the Company all Shares so tendered.

        In the event that more than 500,000 Shares (or such greater number of Shares as the Company may elect to purchase pursuant to the Offer) are properly tendered at or below the Purchase Price and not properly withdrawn, the Company will accept Shares for purchase in the following order of priority, upon the terms and subject to the conditions of the Offer:

•

First, the Company will purchase all "odd lots" of less than 100 Shares at the Purchase Price from shareholders who properly tender all of their Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time; and



•

Second, after purchasing all the "odd lots" that were properly tendered at or below the Purchase Price, the Company will purchase Shares at the Purchase Price from all other holders who properly tender Shares at or below the Purchase Price and who do not properly withdraw them before the Expiration Time, on a pro rata basis, with appropriate adjustments to avoid purchases of fractional Shares, until the Company has acquired the number of Shares that the Company has offered to purchase.

        Because of the "odd lot" priority and proration tender provisions described above, the Company may not purchase all Shares that you tender even if you tender them at or below the purchase price. See Section 1 of the Offer to Purchase.

        WE ARE THE HOLDER OF RECORD OF SHARES HELD FOR YOUR ACCOUNT. A TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND PURSUANT TO YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER YOUR SHARES HELD BY US FOR YOUR ACCOUNT. WE REQUEST INSTRUCTION AS TO WHETHER YOU WISH US TO TENDER ANY OR ALL OF THE SHARES HELD BY US FOR YOUR ACCOUNT UPON THE TERMS AND SUBJECT TO THE CONDITIONS SET FORTH IN THE OFFER TO PURCHASE AND THE LETTER OF TRANSMITTAL.

Your attention is called to the following:

1.

Shares may be tendered at a price not greater than $875 or less than $825 per Share or at the price determined pursuant to the Offer, as indicated in the attached instruction form, net to the seller in cash, less any applicable withholding taxes and without interest, and all shares will be purchased at the same price, regardless of whether a shareholder tendered at a lower price.



2.

You should consult with your broker or other financial or tax advisor on the possibility of designating the priority in which your shares will be purchased in the event of proration.



3.

The Offer is not conditioned upon any minimum number of Shares being properly tendered. However, the Offer is subject to other conditions, which you should read carefully. See Section 8 of the Offer to Purchase.



4.

Tendering shareholders will not be obligated to pay brokerage commissions or, subject to Instruction 6 of the Letter of Transmittal, transfer taxes on the purchase of Shares by the Company pursuant to the Offer.



5.

The Offer, proration period and withdrawal rights expire at 12:00 midnight, New York City time, at the end of the day on September 14, 2017, unless extended. Shares must be properly tendered by the Expiration Time to ensure that at least some of your Shares will be purchased if there is proration. Your instructions to us should be forwarded in ample time to permit us to submit a timely tender on your behalf.



6.

The Offer is for up to 500,000 Shares, constituting approximately 12% of the Company's outstanding Shares as of August 16, 2017.

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7.

If you wish to tender portions of your Shares at different prices, you must complete a separate Instruction Form for each price at which you wish to tender each such portion of your Shares. We must submit separate Letters of Transmittal on your behalf for each price you designate.



8.

If you are an Odd Lot Holder (as defined in the Offer to Purchase) and you instruct us to tender on your behalf all of the Shares that you own at or below the purchase price before the Expiration Time and check the box captioned "Odd Lots" on the attached instruction form, the Company, on the terms and subject to the conditions of the Offer, will accept all such Shares for purchase before proration, if any, of the purchase of other Shares properly tendered at or below the purchase price and not properly withdrawn before the Expiration Time.



9.

None of the Company, our Board of Directors, the Dealer Managers, the Depositary or the Information Agent makes any recommendation as to whether any shareholder should tender Shares pursuant to the Offer or as to the purchase price or prices at which a shareholder may choose to tender them. You must make your own decision after consulting with your own advisors as to whether to tender your Shares and, if so, how many Shares to tender and the price or prices at which you will tender your Shares. In doing so, you should read carefully the information set forth or incorporated by reference in the Offer to Purchase and in the related Letter of Transmittal, including the Company's reasons for making the Offer. See "Section 5. Purpose of the Offer; Certain Effects of the Offer". The directors and executive officers of the Company have advised us that they do not intend to tender any Shares owned by them in the Offer. The Trust under the Executive Survivor Income Plan maintained by our subsidiary, OneBeacon Services, LLC, may elect to tender a number of Shares representing up to an aggregate value of $4.0 million. There can be no assurances that such Trust will tender Shares in the Offer or the price or prices at which any such Shares may be tendered.



10.

The Offer is subject to the terms and conditions set forth in the Offer to Purchase, which you should read carefully.

        If you wish to have us tender any or all of your Shares, please so instruct us by completing, executing, detaching and returning to us the attached Instruction Form. If you authorize us to tender your Shares, we will tender all your Shares unless you specify otherwise on the attached Instruction Form.

        The method of delivery of this document is at the election and the risk of the tendering shareholders. If delivery is by mail, registered mail with return receipt requested, properly insured, is recommended. In all other cases, sufficient time should be allowed to ensure timely delivery.

        YOUR PROMPT ACTION IS REQUESTED. YOUR INSTRUCTION FORM SHOULD BE FORWARDED TO US IN AMPLE TIME TO PERMIT US TO SUBMIT THE TENDER ON YOUR BEHALF BEFORE THE EXPIRATION OF THE OFFER. PLEASE NOTE THAT THE OFFER AND PRORATION PERIOD AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY TIME, AT THE END OF THE DAY ON SEPTEMBER 14, 2017, UNLESS THE OFFER IS EXTENDED.

        The Offer is being made solely pursuant to the Offer to Purchase and the related Letter of Transmittal and is being made to all holders of the Shares. The Offer is not being made to (nor will tenders be accepted from or on behalf of) holders of Shares residing in any jurisdiction in which the making of the Offer or the acceptance thereof would not be in compliance with the securities or Blue Sky laws of such jurisdiction.

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INSTRUCTION FORM WITH RESPECT TO
OFFER TO PURCHASE FOR CASH
by
WHITE MOUNTAINS INSURANCE GROUP, LTD.
of
UP TO 500,000 OF ITS COMMON SHARES
at a Purchase Price not greater than $875
or less than $825 per Share

        The undersigned acknowledge(s) receipt of your letter enclosing the Offer to Purchase dated August 17, 2017, and a specimen Letter of Transmittal relating to the Offer by White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), to purchase 500,000 of its common shares, par value $1.00 per share ("Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest.

        The undersigned hereby instructs you to tender to the Company the number of Shares indicated below (or, if no number is indicated below, all Shares) that are held by you for the account of the undersigned at the price indicated below, upon the terms and subject to the conditions set forth in the Offer to Purchase and in the related specimen Letter of Transmittal that you have furnished to the undersigned.

        In participating in the Offer, the undersigned acknowledges that: (1) the Offer is discretionary and may be extended, modified, suspended or terminated by the Company as provided in the Offer; (2) the undersigned is voluntarily participating in the Offer; (3) the future value of the Company's Shares is unknown and cannot be predicted with certainty; (4) the undersigned has consulted his, her or its tax and financial advisors with regard to how the Offer will impact his, her or its personal situation; (5) any foreign exchange obligations triggered by the undersigned's tender of Shares or the recipient of proceeds are solely his, her or its responsibility; and (6) regardless of any action that the Company takes with respect to any or all income/capital gains tax, social security or other similar tax-related items that are imposed on the undersigned ("Tax Items") related to the Offer and the disposition of Shares, the undersigned acknowledges that the ultimate liability for all Tax Items is and remains his, her or its sole responsibility. In that regard, the undersigned authorizes the Company to withhold, to the extent required by law, all applicable Tax Items legally payable by the undersigned.

        The undersigned consents to the collection, use and transfer, in electronic or other form, of the undersigned's personal data as described in this document by and among, as applicable, the Company, its subsidiaries, and third-party administrators for the exclusive purpose of implementing, administering and managing his, her or its participation in the Offer.

        The undersigned understands that the Company holds certain personal information about him, her or it, including, as applicable, but not limited to, the undersigned's name, home address and telephone number, date of birth, social security or insurance number or other identification number, nationality, any shares of stock held in the Company, details of all options or any other entitlement to shares outstanding in the undersigned's favor, for the purpose of implementing, administering and managing his, her or its stock ownership ("Data"). The undersigned understands that Data may be transferred to any third parties assisting in the implementation, administration and management of the Offer, that these recipients may be located in his, her or its country or elsewhere, and that the recipient's country may have different data privacy laws and protections than his, her or its country. The undersigned understands that he, she or it may request a list with the names and addresses of any potential recipients of the Data by contacting the Company. The undersigned authorizes the recipients to receive, possess, use, retain and transfer the Data, in electronic or other form, for the purposes of implementing, administering and managing his, her or its participation in the Offer, including any requisite transfer of such Data as may be required to a broker or other third party with whom the undersigned holds any shares of stock. The undersigned understands that Data will be held only as long

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as is necessary to implement, administer and manage his, her or its participation in the Offer. The undersigned understands that he, she or it may, at any time, view Data, request additional information about the storage and processing of Data, require any necessary amendments to Data or refuse or withdraw the consents herein, in any case without cost, by contacting in writing the Company. The undersigned understands, however, that refusing or withdrawing his, her or its consent may affect his, her or its ability to participate in the Offer. For more information on the consequences of his, her or its refusal to consent or withdrawal of consent, the undersigned understands that he, she or it may contact the Company.

NUMBER OF SHARES TO BE TENDERED:                          SHARES*

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*

Unless otherwise indicated it will be assumed that all of your Shares are to be tendered.

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CHECK ONLY THE BOX UNDER 1 OR 2 BELOW. IF BOTH BOXES UNDER 1 OR 2 ARE CHECKED, OR IF NEITHER OF THE BOXES

UNDER 1 OR 2 ARE CHECKED, THERE IS NO VALID TENDER OF SHARES.
(See Instruction 7 of the Letter of Transmittal)

1.     SHARES TENDERED AT A PRICE DETERMINED PURSUANT TO THE OFFER

o

By checking this box INSTEAD ONE OF THE BOXES UNDER 2 BELOW, the undersigned hereby tenders Shares and is willing to accept the purchase price determined by the Company pursuant to the Offer. This action will maximize the chance of having the Company purchase all of the Shares tendered by the undersigned (subject to the possibility of proration). The undersigned understands that this election could result in the tendered shares being purchased at the minimum price of $825 per Share, which is below the closing market price for the Shares on August 16, 2017, the last full trading day prior to the public announcement of the Offer, when the closing market price on the NYSE was $850.65.

—OR—

2.     SHARES TENDERED AT A PRICE DETERMINED BY SHAREHOLDER

By checking ONE of the boxes below INSTEAD OF THE BOX UNDER 1 ABOVE, the undersigned hereby tenders Shares at the price checked the corresponding box in the section below captioned "Price (in Dollars) per Share at which Shares are Being Tendered". This action could result in none of the Shares being purchased if the purchase price for the Shares is less than the price checked. If the purchase price for the Shares is equal to or greater than the price checked, then the Shares purchased by the Company will be purchased at the purchase price. All Shares so purchased by the Company will be purchased at the same price regardless of whether the shareholder tendered at a lower price. A shareholder who wishes to tender Shares at more than one price must complete a separate Letter of Transmittal for each price at which Shares are being tendered. The same Shares cannot be tendered (unless previously withdrawn in accordance with the terms of the Offer) at more than one price.

PRICE (IN DOLLARS) PER SHARE AT WHICH SHARES ARE BEING TENDERED.

CHECK ONLY ONE BOX BELOW. IF MORE THAN ONE BOX BELOW IS CHECKED OR IF NO BOX BELOW IS CHECKED, THERE IS NO VALID TENDER OF SHARES.

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ODD LOTS
(See Instruction 8 of the Letter of Transmittal)

        As described in Section 1 of the Offer to Purchase, under certain conditions, a shareholder owning beneficially or of record an aggregate of fewer than 100 Shares and who tenders all such Shares and satisfies the other requirements set forth in the Offer to Purchase, may have his or her Shares accepted for payment before any proration of other tendered Shares. This preference is not available to partial tenders, or to beneficial or record holders of an aggregate of 100 or more Shares, even if these holders have separate accounts representing fewer than 100 Shares. Accordingly, this section is to be completed only if Shares are being tendered by or on behalf of a person owning, beneficially or of record, an aggregate of fewer than 100 Shares. The undersigned either (check one box):

o

owns, whether beneficially or of record, an aggregate of fewer than 100 Shares and is tendering all such Shares.

        In addition, the undersigned is tendering all such Shares either (check one box):

o

at the Purchase Price, as the same will be determined by the Company in accordance with the terms of the Offer (persons checking this box should not check any box in the section captioned "Price (in Dollars) per Share at which Shares are Being Tendered"); or

o

at the price per Share indicated above in the section captioned "Price (in Dollars) per Share at which Shares are Being Tendered".

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Exhibit (a)(1)(E)

OFFER TO PURCHASE FOR CASH by WHITE MOUNTAINS INSURANCE GROUP, LTD. of UP TO 500,000 OF ITS COMMON SHARES at a Purchase Price not greater than $875 or less than $825 per Share
NUMBER OF SHARES TO BE TENDERED: SHARES
ODD LOTS (See Instruction 8 of the Letter of Transmittal)






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Exhibit (a)(1)(G)

August 17, 2017

Dear Fellow Shareholder:

        White Mountains Insurance Group, Ltd. (the "Company") is offering to purchase 500,000 of its common shares, or such lesser number of its common shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per share, net to the seller in cash, less any applicable withholding taxes and without interest. The Company's common shares closed on the New York Stock Exchange at $850.65 per share on August 16, 2017, the last trading day before we announced the offer.

        The tender offer will be conducted as a "modified Dutch auction", pursuant to which we will select the single lowest per share price (in increments of $1.00) that will allow us to purchase the number of shares sought in the offer (or, if a lesser number is properly tendered, all shares that are properly tendered and not properly withdrawn). All shares acquired in the offer will be acquired at the same price regardless of whether the shareholder tendered at a lower price. However, because of the proration and "odd lot" priority provisions described in the enclosed Offer to Purchase, all of the shares tendered at or below the purchase price may not be purchased if more than the number of shares we seek are properly tendered. All shares tendered by you that we do not purchase will be returned to you at our expense promptly after the expiration of the offer.

        The offer is being made to utilize a portion of the Company's undeployed capital and to provide shareholders with added liquidity. The offer represents the opportunity for us to return capital to shareholders who elect to tender their shares. The offer will also afford to shareholders the opportunity to dispose of shares without the usual transaction costs associated with any market sale. Shareholders whose shares are not purchased in the offer will obtain an increase in their ownership interest in the Company and thus in the Company's future earnings and assets because shares purchased pursuant to the offer will be cancelled.

        The offer is explained in detail in the enclosed Offer to Purchase and Letter of Transmittal. If you want to tender your shares, the instructions for tendering shares are also explained in detail in the enclosed materials. This letter is only a summary, and I encourage you to read these materials carefully before making any decision with respect to the offer. The instructions on how to tender shares are explained in detail in the accompanying materials.

        OUR BOARD OF DIRECTORS HAS APPROVED THE OFFER. HOWEVER, NONE OF THE COMPANY, OUR BOARD OF DIRECTORS, THE DEALER MANAGERS, THE DEPOSITARY OR THE INFORMATION AGENT MAKES ANY RECOMMENDATION AS TO WHETHER ANY SHAREHOLDER SHOULD TENDER OR REFRAIN FROM TENDERING SHARES PURSUANT TO THE OFFER, OR AS TO THE PRICE OR PRICES AT WHICH A SHAREHOLDER MAY CHOOSE TO TENDER. YOU MUST MAKE YOUR OWN DECISION AFTER CONSULTING WITH YOUR ADVISORS WHETHER TO TENDER SHARES AND, IF SO, HOW MANY SHARES TO TENDER AND THE PRICE OR PRICES AT WHICH YOU WILL TENDER THEM. IN DOING SO, YOU SHOULD READ CAREFULLY THE INFORMATION SET FORTH OR INCORPORATED BY REFERENCE IN THE OFFER TO PURCHASE AND IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING THE PURPOSES AND EFFECTS OF THE OFFER.

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        The offer will expire at 12:00 midnight, New York City time, at the end of the day on September 14, 2017, unless the Company extends the offer. Questions and requests for assistance may be directed to D.F. King & Co., Inc., the information agent for the offer, at the address, telephone numbers or email address set forth on the back cover of the Offer to Purchase. You may request additional copies of the Offer to Purchase and other offer documents from the information agent at the telephone number and address on the back cover of the Offer to Purchase.

        This communication is for information purposes only and does not constitute an offer to buy or the solicitation of an offer to sell shares of the Company's common stock. The Company's offer to buy the Company's common shares is being made only pursuant to the enclosed Offer to Purchase and the related materials dated August 17, 2017, as amended and supplemented from time to time. Shareholders should read the Offer to Purchase and the related materials carefully because they contain important information. You may also obtain a free copy of the tender offer statement on Schedule TO, the Offer to Purchase and other documents filed with the Securities and Exchange Commission at the Commission's website at www.sec.gov.

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Exhibit (a)(1)(G)

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Exhibit (a)(1)(H)

LETTER TO PARTICIPANTS IN THE
ONEBEACON 401(K) SAVINGS AND EMPLOYEE STOCK OWNERSHIP PLAN

August 17, 2017

Dear Plan Participant:

        White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), is offering to purchase 500,000 of its common shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase (as it may be amended or supplemented from time to time, the "Offer to Purchase") and in the related Letter of Transmittal (which together, as they may be amended or supplemented from time to time, constitute the "Offer"), including the proration provisions described therein. The Shares closed on the New York Stock Exchange at $850.65 per share on August 16, 2017, the last trading day before we announced the Offer.

        The Offer will be conducted as a "modified Dutch auction", pursuant to which the Company will select the lowest price per Share (in increments of $1.00) (the "Purchase Price") that will allow the Company to purchase the number of Shares sought in the Offer or, if a lesser number is properly tendered, all Shares that are properly tendered and not properly withdrawn. All Shares acquired in the Offer will be acquired at the same price regardless of whether the shareholder tendered at a lower price. However, because of the proration and "odd lot" priority provisions described in the Offer to Purchase, fewer than all of the Shares tendered at or below the Purchase Price may be purchased if more than the number of Shares the Company seeks are properly tendered and not properly withdrawn. Shares tendered but not purchased in the Offer, including Shares tendered at or below the Purchase Price and Shares not purchased because of proration, will be returned to the tendering shareholders at the Company's expense promptly after the expiration of the Offer.

        A copy of each of the Offer to Purchase, the 401(k) Plan Participant Q&A, the 401(k) Plan Participant Election Form (the "Election Form") and the 401(k) Plan Participant Notice of Withdrawal (the "Notice of Withdrawal") is enclosed with this letter. You are urged to examine the Offer carefully.

        As a participant in the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (the "Plan"), you have the right to instruct Vanguard Fiduciary Trust Company, the Trustee of the Plan (the "Trustee") via communication with Computershare Trust Company, N.A., as agent (the "Agent") for the benefit of the Trustee, as to whether to tender any Shares allocated to your Plan account ("Plan Shares"). If, after reading the enclosed materials, you want to tender your Plan Shares, you must follow the instructions contained in this letter and fill out and sign the Election Form enclosed, and mail the form in the enclosed self-addressed envelope to the Agent in connection with the Offer. If you do not wish to use the enclosed envelope, you may also send your Election Form by express or overnight delivery to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, 250 Royall Street, Suite V, Canton, Massachusetts 02021, or by first-class, registered or certified mail to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, PO Box 43011, Providence, Rhode Island 02940-3011.

        If valid instructions to tender Plan Shares are not received by 5:00 p.m., New York City time, on September 12, 2017, the Plan Shares allocated to your Plan account will not be tendered unless the Company extends the Offer, in which case your instructions must be received by 5:00 p.m., New York City time, on the date that is two (2) business days before the new expiration date. Please note that your instructions as to whether or not to tender will be kept confidential from the Company and OneBeacon Services, LLC (the "Plan Sponsor").

        If you elect to tender Plan Shares and such Plan Shares are accepted in the Offer, any proceeds received in respect of such Plan Shares will remain in the Plan and will be invested in the Plan's Qualified Default Investment Alternative, which is the Vanguard Target Retirement Fund closest to the year you will turn age 65. However, you may elect to redirect the proceeds to any other investment

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fund under the Plan that is open for new investments at any time once the proceeds have been allocated to your account under the Plan.

        Whether you decide to tender your Plan Shares or not, all Plan Shares in the White Mountains ESOP Fund (the "ESOP Fund") in the Plan will be "frozen" for two business days. Transactions involving the ESOP Fund in the Plan will not be permitted starting at 1:00 p.m. Eastern time on Tuesday, September 12, 2017, unless the Offer is extended or withdrawn prior to that time. If the Offer is extended, the ESOP Fund will be "frozen" starting at 1:00 p.m. Eastern time on the date that is two business days before any new expiration date.

        If you choose to instruct the Trustee to tender your Plan Shares, certain transactions involving the Plan, including all withdrawals, distributions and transfers, will be prohibited until the earlier of such time as: (i) the Offer is terminated and all Plan Shares are returned to the Trustee or (ii) the Offer is consummated and the Plan Shares are purchased by the Company. These restrictions on transactions will apply only to Plan Shares tendered into the Offer and will not apply to any Plan Shares not tendered into the Offer. In the event that the Offer is extended, the restrictions on transactions involving the Plan will continue until the consummation or expiration of the Offer, as extended.

        You must direct the Trustee if you want to tender your Plan Shares and, if you direct the Trustee to tender your Plan Shares, you must indicate at which price or prices you want the Trustee to tender them. The Trustee will tender your Plan Shares only if specifically instructed to do so.

        If you do not respond using the enclosed Election Form, you will be deemed to have instructed the Trustee not to tender any of your Plan Shares under the Offer, and your Plan Shares will remain in your Plan account.

        If you are also a direct shareholder of the Company, you will receive under separate cover another copy (or copies) of the Offer documents which can be used to tender your directly held Shares if you choose to do so. Instructions with respect to tendering your directly held Shares will be set forth in those materials. Those Offer documents may not be used to direct the Trustee to tender or not to tender the Plan Shares allocable to your individual account under the Plan.

        Please note that the Trustee and the Plan are prohibited by law from selling Plan Shares to the Company for a price that is less than the prevailing market price of the Shares. Accordingly, if you elect to tender Plan Shares at a price that is lower than the closing price of the Shares on the date the Offer expires, the tender price you elect will be deemed to have been increased to the closest tender price that is not less than the closing price of the Shares on the New York Stock Exchange on the date the Offer expires. This could result in none of your Plan Shares being accepted for purchase by the Company. Similarly, if you elect to maximize the chance of having the Company purchase Plan Shares by instructing the Trustee to tender by checking Box 2 of the Election Form and writing a percentage on the "% at TBD" line on the Election Form and the closing price of the Shares on the New York Stock Exchange on the date the Offer expires is within the range of prices set forth on the Election Form, the tender price you elect will be deemed to have been increased to the closest tender price that is not less than the closing price of the Shares on the New York Stock Exchange on the date the Offer expires. If the closing price of the Shares on the date the Offer expires is greater than the maximum price available in the Offer, none of the Plan Shares will be tendered and your tender instruction will be deemed to have been withdrawn.

        You also may request that the Trustee withdraw any tender instruction you have previously submitted, as long as you do so prior to 5:00 p.m., New York City time, on September 12, 2017, by delivering the Notice of Withdrawal to the Agent. The Notice of Withdrawal cannot be sent by facsimile. Any Notice of Withdrawal that is sent by facsimile to the Agent will not be considered valid. If you do not have a copy of the Notice of Withdrawal, you may also give your withdrawal instructions (which must contain the information set forth below) by letter to the Agent. If the Offer is extended and you wish to withdraw your previously submitted tender instruction, then you must ensure that the Agent receives your Notice of Withdrawal or other withdrawal notice by 5:00 p.m., New York City time, on the date that is two (2) business days before the new expiration date. The Notice of Withdrawal

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sent to the Agent must: (i) specify the name of the participant or beneficiary who has made the instruction that is being withdrawn and the participant's or beneficiary's social security number and (ii) be signed by the participant or beneficiary who signed the Election Form.

        If you wish to withdraw any tender instructions for Plan Shares, you must withdraw the tender instructions for all Plan Shares that have been submitted into the Offer. You may instruct the Trustee to re-tender any or all of these Plan Shares into the Offer by submitting a new Election Form by mail.

        For any Plan Shares that are tendered and purchased by the Company, the Company will pay cash to the Plan. Individual participants in the Plan will not, however, receive any cash tender proceeds directly. All such proceeds will remain in the Plan and may be withdrawn only in accordance with the terms of the Plan.

        None of the Trustee, the Agent, the Company, the Plan Sponsor, the Company's Board of Directors, Computershare Trust Company, N.A., which is acting as the depositary (in such capacity, the "Depositary"), Barclays Capital Inc. and J.P. Morgan Securities LLC, which are severally acting as the dealer managers or D.F. King & Co., Inc. ("D.F. King"), which is acting as the information agent (the "Information Agent"), makes any recommendation as to whether you should tender or refrain from tendering your Plan Shares or as to the price or prices at which you should tender your Plan Shares. You must make your own decision after consulting with your own advisors whether to tender your Plan Shares and, if so, how many Plan Shares to tender and the price or prices at which you may choose to tender your Plan Shares.

        If you have any questions about the Offer, you should contact D.F. King, the Information Agent for the Offer, toll-free at (800) 893-5865, collect at (212) 269-5550 or by email at wtm@dfking.com. Additionally, all tender offer materials are available online at www.sec.gov. If you have questions about your Plan account or questions about how to provide tender instructions to Vanguard, please contact Vanguard Participant Services at (800) 523-1188 Monday through Friday from 8:30 a.m. to 9 p.m., Eastern time.

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QUESTIONS AND ANSWERS WITH RESPECT TO
TENDER RIGHTS OF PARTICIPANTS IN
THE ONEBEACON 401(K) SAVINGS AND EMPLOYEE STOCK OWNERSHIP PLAN

DESCRIPTION OF THE OFFER

1.

What is the Offer?

        White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the "Company"), is offering to purchase 500,000 of its Common Shares, par value $1.00 per share (the "Shares"), or such lesser number of Shares as are properly tendered and not properly withdrawn, at a purchase price not greater than $875 or less than $825 per Share, net to the seller in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase (as it may be amended or supplemented from time to time, the "Offer to Purchase") and in the related Letter of Transmittal (which together, as they may be amended or supplemented from time to time, constitute the "Offer"), including the proration provisions described therein. The Company will select the lowest price per Share (in increments of $1.00) (the "Purchase Price") that will allow the Company to purchase the number of Shares sought in the Offer or, if a lesser number is properly tendered, all Shares that are properly tendered and not properly withdrawn. All Shares acquired in the Offer will be acquired at the same price regardless of whether the shareholder tendered at a lower price. However, because of the proration and "odd lot" priority provisions described in the Offer to Purchase, fewer than all of the Shares tendered at or below the Purchase Price may be purchased if more than the number of Shares the Company seeks are properly tendered and not properly withdrawn. Shares tendered but not purchased in the Offer, including Shares tendered at or below the Purchase Price and Shares not purchased because of proration, will be returned to the tendering shareholders at the Company's expense promptly after the expiration of the Offer.

        Participants in the OneBeacon 401(k) Savings and Employee Stock Ownership Plan (the "Plan") who have equivalent Shares allocated to their Plan accounts (such Shares are referred to in this Q&A as "Plan Shares") may tender these Plan Shares by filling out and signing the Election Form enclosed, and mailing the form in the enclosed self-addressed envelope to Computershare Trust Company, N.A., as agent (the "Agent") for the benefit of Vanguard Fiduciary Trust Company (the "Trustee"). If you do not wish to use the enclosed envelope, you may also send your Election Form by express or overnight delivery to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, 250 Royall Street, Suite V, Canton, Massachusetts 02021, or by first-class, registered or certified mail to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, PO Box 43011, Providence, Rhode Island 02940-3011.

        You should mail your completed Election Form to the Agent no later than 5:00 p.m., New York City time, on September 12, 2017, or, if the Offer is extended, by 5:00 p.m., New York City time, on the date that is two (2) business days before the new expiration date.

        After the Offer expires, the Company will look at the prices chosen by shareholders for all Shares properly tendered and not withdrawn. The Company will then select the lowest Purchase Price (in increments of $1.00) within the price range specified above that will allow the Company to purchase 500,000 Shares, or such lesser number of Shares as are properly tendered and not properly withdrawn. All Shares that the Company acquires in the Offer will be acquired at the same Purchase Price regardless of whether the shareholder tendered at a lower price. The Company will purchase only Shares tendered at prices at or below the Purchase Price it determines. However, because of the "odd lot" priority and proration provisions described in the Offer to Purchase (which apply to Shares other than Plan Shares), the Company may not purchase all of the Shares tendered at or below the Purchase Price if, based on the Purchase Price it determines, more than 500,000 Shares are properly tendered and not properly withdrawn. Shares tendered but not purchased in the Offer will be returned to the tendering shareholders at the Company's expense promptly after the expiration of the Offer. See Section 2 of the Offer to Purchase.

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        In the event that more than 500,000 Shares are tendered in the Offer, the Company may exercise its right to increase the number of Shares sought in the Offer by an amount not exceeding 2% of its outstanding Shares without extending the expiration date. The Company also expressly reserves the right, in its sole discretion, to amend the Offer to purchase additional Shares, subject to applicable law.

        If more than 500,000 Shares (or such greater number of Shares as we may elect to purchase, subject to applicable law) are properly tendered at or below the Purchase Price and not properly withdrawn, the Company will purchase Shares in the following order of priority:

•

First, the Company will purchase all "odd lots" of less than 100 Shares at the Purchase Price from shareholders who properly tender all of their Shares at or below the Purchase Price and who do not properly withdraw them before the expiration date; and



•

Second, after purchasing all the "odd lots" that were properly tendered at or below the Purchase Price, the Company will purchase Shares at the Purchase Price from all other holders who properly tender Shares at or below the Purchase Price and who do not properly withdraw them before the expiration date, on a pro rata basis, with appropriate adjustments to avoid purchases of fractional Shares, until we have acquired the number of Shares that we have offered to purchase.

        Therefore, the Company may not purchase all of the Shares that you tender even if you tender them at or below the Purchase Price.

        The terms and conditions of the Offer are fully described in the enclosed Offer to Purchase. Please read the Offer to Purchase carefully.

2.

What are my rights under the Offer?

        The Plan records indicate that Plan Shares are allocated to your account. You may tender some or all of these Plan Shares. Because the Employee Retirement Income Security Act of 1974, as amended ("ERISA"), requires that all Plan assets in most circumstances be held in trust, all of these Plan Shares are registered in the name of Vanguard Fiduciary Trust Company. Consequently, the Trustee will actually tender your Plan Shares by aggregating all Plan participant instructions and completing the required letter of transmittal for all Plan participants, but only in accordance with your instructions as well as those of the other Plan participants.

        You must direct the Trustee if you want to tender your Plan Shares and, if you direct the Trustee to tender your Plan Shares, you must indicate at which price or prices you want the Trustee to tender them. The Trustee will tender your Plan Shares only if specifically instructed to do so. If you do not respond using the enclosed Election Form, you will be deemed to have instructed the Trustee not to tender any of your Plan Shares under the Offer, and your Plan Shares will remain in your Plan account.

        The Agent is acting on behalf of the Trustee in connection with the Offer.

        Plan Shares will not qualify for the "odd lot" preference described above and may not be conditionally tendered. See Section 1 of the Offer to Purchase for a description of the "odd lot" preference.

3.

Which documents did I receive in the Offer materials and what is the purpose of each document?

        The following materials are enclosed in this mailing:

(a)

Letter from the Trustee.    This letter gives you details for participating in the Offer with respect to the Plan Shares in your Plan account.



(b)

Offer to Purchase dated August 17, 2017.    This document describes all of the terms and conditions of the Offer. Please read this document carefully.

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(c)

Election Form.    You must complete, sign and mail this document to the Agent if you wish to direct the Trustee to tender some or all of your Plan Shares. If your fully completed Election Form is not received by the Agent by 5:00 p.m., New York City time, on September 12, 2017, you will be deemed to have instructed the Trustee not to offer any of your Plan Shares for sale under the Offer. You must use the Election Form if you wish to direct a tender of your Plan Shares.



(d)

Reply Envelope.    A self-addressed envelope is provided for your convenience. If you decide to tender some or all of your Plan Shares, you may use this envelope to mail the completed Election Form to the Agent.



(e)

Notice of Withdrawal.    This form can be used to withdraw Plan Shares tendered into the Offer. See Q&A #11 below for additional information.

4.

Will I need to comply with the Company's policy or with OneBeacon Insurance Group, Ltd.'s policy regarding pre-clearance of sales in order to tender my Plan Shares in the Offer?

        You will not need pre-clearance in order to tender your Plan Shares in the Offer.

5.

How do I direct the Plan Trustee?

        The only way to instruct the Trustee to tender your Plan Shares is by completing the Election Form as described, signing it and returning it to the Agent.

        If you choose to mail your Election Form to the Agent, you may do this by using the self-addressed envelope provided for this purpose. Alternatively, you may send your Election Form by first-class, registered or certified mail or express or overnight delivery to the Agent.

        The Agent must receive your Election Form before 5:00 p.m., New York City time, on September 12, 2017 (unless the Offer is extended, in which case such Election Form must be received by no later than 5:00 p.m., New York City time, on the date that is two (2) business days before the new expiration date). If you send the Agent the Election Form, you must sign and complete the Election Form for your tender instruction to be valid.

        To validly direct the Trustee to Tender Plan Shares on your behalf by mailing the Election Form, you must:

(a)

Instructions.    Read carefully and follow exactly the instructions in (i) the Letter from the Trustee and (ii) the Election Form. These documents will tell you how to direct the Trustee regarding your Plan Shares.



(b)

Election Form.    Complete the enclosed Election Form.



(c)

Shares.    Designate on the Election Form the percentage of your Plan Shares you wish to be tendered.



(d)

Price.    Designate on the Election Form the price or prices at which you are willing to tender your Plan Shares. In the alternative, you may maximize the chance of the Company purchasing the Plan Shares you tender by electing to accept whatever price the Company determines as the Offer Purchase Price pursuant to the Offer terms and conditions. Please note that the latter action may result in your receiving a price as low as $825 per Share.



(e)

Signature.    You must SIGN the Election Form to complete your instruction. Unless you sign the Election Form, your direction cannot be honored, and the Election Form will be ineffective even if it is timely received by the Trustee.



(f)

Mailing.    We have enclosed a self-addressed return envelope with your tender materials. You may use this self-addressed envelope to return your completed Election Form if you wish to have the Trustee tender your Plan Shares. If you do not wish to use the enclosed envelope, you may also send your Election Form by first-class, registered or certified mail to the Agent

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at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, PO Box 43011, Providence, Rhode Island 02940-3011, or by express or overnight delivery to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, 250 Royall Street, Suite V, Canton, Massachusetts 02021.

        Please be precise in providing your instruction and please act promptly. Do not deliver your instructions to your Human Resources Department or The Vanguard Group, Inc. ("Vanguard").

        If you do not wish to tender any Plan Shares, take no action.

6.

Must I provide directions to the Trustee?

        You must respond to the Agent only if you wish the Trustee to tender any of your Plan Shares. If you do not wish to tender any of your Plan Shares, do nothing. If you do nothing, you will be deemed to have instructed the Trustee not to tender any of the Plan Shares held for your benefit. A failure to respond to this tender offer will be deemed as an instruction not to tender your Plan Shares.

7.

How many Plan Shares may I tender and how do I learn the number of Plan Shares held for my benefit in the Plan?

        You may tender a whole percentage (up to 100%) of the Plan Shares allocated to your Plan account as of the expiration date of the Offer, currently scheduled to occur on September 14, 2017 (unless it is extended). The approximate number of Plan Shares held in your Plan account is calculated by dividing the value of your Plan account allocated to the White Mountains ESOP Fund under the Plan by the New York Stock Exchange closing price of the Company's common shares on a particular day. Please note that this is only an estimate of the number of Plan Shares allocated to your Plan account, as the White Mountains ESOP Fund includes both Shares and a small portion of cash. You may obtain information about the number of Plan Shares allocated to your Plan account, or confirm your current balance, by contacting Vanguard at (800) 523-1188 or www.vanguard.com for assistance.

        If you wish to tender portions of your Plan Shares at different prices, you must identify the whole percentage of your Plan Shares you wish to tender at each specific price at which you wish to instruct the Trustee to tender your Plan Shares.

8.

Why must I direct the tender of Plan Shares allocated to my Plan account by whole percentage, rather than designating a set number of Plan Shares?

        A whole percentage designation allows the Trustee to take into account transactions involving Plan Shares that might be effected after you complete and send your Election Form to the Agent, such as additional contributions to investment funds under the Plan that are open for new investments, exchanges or distributions of Plan Shares. The percentage designation allows the Trustee to tender your Plan Shares based on the actual number of Shares in your Plan account as of the date of such tender.

9.

What if I have Shares in my Plan account and hold Shares outside of the Plan?

        If you have Shares in the Plan and also own other Shares (either in your possession or held by a bank or brokerage firm, or otherwise) outside of the Plan, you will receive two or more sets of Offer materials. You should be careful to follow the different instructions that apply to tendering each kind of Shares.

10.

Who will know whether I tendered my Plan Shares?

        Your directions to the Agent for the benefit of the Trustee will be kept confidential. No employee, officer or director of the Company will learn of your instruction unless such disclosure is required by law.

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11.

Can I change my mind and direct the Trustee to withdraw Plan Shares that I previously directed the Trustee to tender?

        Yes, but only if you perform all of the following steps:

(a)

The Notice of Withdrawal cannot be sent by facsimile. Any Notice of Withdrawal that is sent by facsimile to the Agent will not be considered valid;



(b)

You must send a completed and signed copy of the enclosed Notice of Withdrawal or other withdrawal notice to the Agent. You may submit a Notice of Withdrawal or other withdrawal notification by first-class, registered or certified mail to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, PO Box 43011, Providence, Rhode Island 02940-3011, or by express or overnight delivery to the Agent at Computershare Trust Company, N.A., c/o Voluntary Corporate Actions, 250 Royall Street, Suite V, Canton, Massachusetts 02021;



(c)

The Notice of Withdrawal or other withdrawal notice must set forth your name and social security number and it must state that you are directing the Trustee to withdraw all Plan Shares that you previously directed the Trustee to tender on your behalf; and



(d)

The Agent must receive the Notice of Withdrawal or other withdrawal notice before 5:00 p.m., New York City time, on September 12, 2017 (unless the Offer is extended, in which case such notice must be received by no later than 5:00 p.m., New York City time, on the date that is two (2) business days before the new expiration date).

        If you wish to withdraw any tender instructions for Plan Shares with respect to the Offer, you must withdraw the tender instructions for all Plan Shares that have been submitted into the Offer. You may instruct the Trustee to re-tender any or all of these Plan Shares into the Offer by submitting a new Election Form by mail to the Agent.

12.

Will the Company purchase all Plan Shares that I direct the Trustee to tender?

        The answer to this question depends on the total number of Shares properly tendered (and not properly withdrawn) by all tendering shareholders at or below the Purchase Price, and the price or prices at which you direct the Trustee to tender your Shares. If you tender your Plan Shares at a price above the Purchase Price determined by the Company pursuant to the terms and conditions of the Offer, the Company will not purchase your Plan Shares. If you tender your Plan Shares at or below the Purchase Price, then the Company will purchase your Plan Shares, subject to the "odd lot" priority and proration provisions of the Offer (which apply to Shares other than Plan Shares). See Q&A #1 for a description of how the "odd lot" priority provision and the proration process work.

        Please note that the Trustee and the Plan are prohibited by law from selling Plan Shares to the Company for a price that is less than the prevailing market price of the Shares. Accordingly, if you elect to tender Plan Shares at a price that is lower than the closing price of the Shares on the date the Offer expires, the tender price you elect will be deemed to have been increased to the closest tender price that is not less than the closing price of the Shares on the New York Stock Exchange on the date the Offer expires. This could result in none of your Plan Shares being accepted for purchase by the Company. Similarly, if you elect to maximize the chance of having the Company purchase Plan Shares by instructing the Trustee to tender by checking Box 2 of the Election Form and writing a percentage on the "% at TBD" line on the Election Form and the closing price of the Shares on the New York Stock Exchange on the date the Offer expires is within the range of prices set forth on the Election Form, the tender price you elect will be deemed to have been increased to the closest tender price that is not less than the closing price of the Shares on the New York Stock Exchange on the date the Offer expires. If the closing price of the Shares on the date the Offer expires is greater than the maximum price available in the Offer, none of the Plan Shares will be tendered and your tender instruction will be deemed to have been withdrawn.

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        Plan Shares held in your Plan account that are tendered but not purchased by the Company will remain in your Plan account as if nothing had happened, subject to the rules and provisions governing the Plan.

13.

What if I have questions about the Offer relating to the Plan?

        If you have any questions about the Offer, please contact D.F. King, the Information Agent for the Offer, at (800) 893-5865 (toll free) , collect at (212) 269-5550 or by email at wtm@dfking.com. Additionally, all tender offer materials are available online at www.sec.gov. If you have questions about your Plan account or questions about how to provide tender instructions to Vanguard, please contact Vanguard Participant Services at (800) 523-1188 Monday through Friday from 8:30 a.m. to 9 p.m., Eastern time.

14.

How will I know if the Company has purchased my Plan Shares?

        The purchase will be reflected in your Plan account as a transfer of the tendered Plan Shares. The tender proceeds will not be in your account until six (6) to ten (10) business days after the Offer expires. At that time, you will be able to see the proceeds in your account on www.vanguard.com, and they will be reflected on your next quarterly statement.

OPERATION OF THE PLAN DURING THE OFFER

15.

What happens to contributions to my Plan account that are made after September 12, 2017?

        Contributions made to your Plan account after September 12, 2017 will be allocated as usual, in accordance with the sources of the contributions and, where applicable, your investment directions in effect at the time of your contribution, including any direction to invest such contributions in Plan Shares. Contributions will continue to be allocated during the Offer, and all of the Plan Shares in your Plan account on September 14, 2017 at 5:00 p.m., New York City time, will be subject to the Offer, unless the Offer is extended, in which case only the Plan Shares in your Plan account at 5:00 p.m., New York City time, on the expiration date will be subject to the Offer.

16.

How will my Plan account be affected by the Offer?

        You will be temporarily unable to conduct certain Plan transactions involving Plan Shares in your Plan account prior to the expiration of Offer. Please review the Plan Letter (as defined below) for more information.

17.

What happens if I request a distribution, withdrawal or transfer following the announcement of the Offer, but before I instruct the Trustee to tender my Plan Shares?

        Distributions and withdrawals from the Plan and transfers into or out of your Plan account will be processed in accordance with normal procedures except for Plan Shares that you have instructed the Trustee to tender into the Offer. In addition, as explained in the Plan Letter, all Plan Shares will be temporarily frozen two business days prior to the expiration date of the Offer.

18.

Will I be taxed on any proceeds received in 2017 from the Shares that I tender from my Plan account?

        No. Because tender proceeds received from Plan Shares will be received by and held in your Plan, they will not be subject to current income taxes.

REINVESTMENT OF OFFER PROCEEDS

19.

How will the Plan invest the proceeds received from the Plan Shares that are tendered?

        If you elect to tender Plan Shares and such Plan Shares are accepted in the Offer, any proceeds received in respect of such Plan Shares will remain in the Plan and will be invested in the Plan's Qualified Default Investment Alternative, which is the Vanguard Target Retirement Fund closest to the year you will turn age 65. Once the proceeds are in your Plan account, you will be able to move such tender proceeds at your own discretion to other investment funds of your choosing within the Plan.

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ELECTION FORM
OneBeacon 401(k) Savings and Employee Stock Ownership Plan (the "Plan")
Your election must be received no later than 5:00 p.m.,
New York City time, on September 12, 2017,
to be included in the tabulation, unless the Offer is extended.

        In accordance with the White Mountains Insurance Group, Ltd. (the "Company") Offer to Purchase dated August 17, 2017 (the "Offer to Purchase"), a copy of which I have received, I hereby instruct Vanguard Fiduciary Trust Company (the "Trustee") to tender or not to tender shares of the Company, par value $1.00 per share (the "Shares"), allocated to my Plan account ("Plan Shares") prior to the Expiration Date (as defined in the Offer to Purchase), as follows:

1.

If you do NOT want to tender any your Plan Shares, do NOT return this form. If you do nothing, none of your Plan Shares will be tendered, unless otherwise required by law or the terms of the Plan.

2.

By writing a whole percentage of Plan Shares allocated to my Plan account in one or more of the spaces below, I elect to tender Plan Shares at the price or prices indicated. This action could result in none of the Plan Shares being purchased if the purchase price for the Plan Shares is less than the price or prices selected. If the purchase price for the Plan Shares is equal to or greater than the price or prices selected, then the Plan Shares purchased by the Company will be purchased at the purchase price indicated, subject to the terms of the Offer to Purchase.

3.

By writing a whole percentage on the "% at TBD" line, I WANT TO MAXIMIZE the chance of having the Company purchase all of the Plan Shares I have instructed the Trustee to tender. Accordingly, by writing a whole percentage on the "% at TBD" line, I am willing to accept the purchase price determined by the Company pursuant to the Offer (as defined in the Offer to Purchase). I understand that this action could result in my receiving a price per Share as low as $825.

Note: The sum of the whole percentages you write in must not exceed 100%. If the sum of all such whole percentages exceeds 100%, none of the Plan Shares allocated to your account will be tendered.

VOLUNTARY CORPORATE ACTION COY: WTM VANGAURD 401(k) PLAN

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        You must direct the Trustee if you want to tender your Plan Shares and, if you direct the Trustee to tender your Plan Shares, you must indicate at which price or prices you want the Trustee to tender them. The Trustee will tender your Plan Shares only if the Trustee is specifically instructed to do so.

        If you do not submit this Election Form to the Agent, who is acting on behalf of the Trustee in connection with the Offer, you will be deemed to have instructed the Trustee not to tender any of your Plan Shares under the Offer, and your Plan Shares will remain in your Plan account.

        If you submit more than one Election Form, the Trustee will rely on the last instructions received from you. The later set of instructions will revoke all prior instructions given to the Trustee with respect to the Offer. Please see the letter from Vanguard Fiduciary Trust Company to participants in the OneBeacon 401(k) Savings and Employee Stock Ownership Plan, dated August 17, 2017 (the "Plan Letter"), for additional information on revoking previously submitted instructions.

        Because you are designated a "named fiduciary" for tender offer purposes under your Plan account, the Trustee is required to follow your validly delivered instructions, provided they are in accordance with the terms of the Plan and are not inconsistent with the Trustee's responsibilities under the Employee Retirement Income Security Act of 1974, as amended ("ERISA"). By signing, dating and returning this instruction form, you accept this designation under the Plan as a "named fiduciary", and that the adjustment to your instructions described below may be made by the Trustee.

        Please note that the Trustee and the Plan are prohibited by law from selling Plan Shares to the Company for a price that is less than the prevailing market price of the Shares. Accordingly, if you elect to tender Plan Shares at a price that is lower than the closing price of the Shares on the date the Offer expires, the tender price you elect will be deemed to have been increased to the closest tender price that is not less than the closing price of the Shares on the New York Stock Exchange on the date the Offer expires. This could result in none of your Plan Shares being accepted for purchase by the Company. Similarly, if you elect to maximize the chance of having the Company purchase Plan Shares by instructing the Trustee to tender by checking Box 2 of the Election Form and writing a percentage on the "% at TBD" line on the Election Form and the closing price of the Shares on the New York Stock Exchange on the date the Offer expires is within the range of prices set forth on the Election Form, the tender price you elect will be deemed to have been increased to the closest tender price that is not less than the closing price of the Shares on the New York Stock Exchange on the date the Offer expires. If the closing price of the Shares on the date the Offer expires is greater than the maximum price available in the Offer, none of the Plan Shares will be tendered and your tender instruction will be deemed to have been withdrawn.

[LABEL WITH PARTICIPANT'S
NAME AND ADDRESS]

VOLUNTARY CORPORATE ACTION COY: WTM VANGAURD 401(k) PLAN

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NOTICE OF WITHDRAWAL FROM THE TENDER OFFER

WHITE MOUNTAINS INSURANCE GROUP, LTD. TENDER OFFER
FOR PARTICIPANTS IN ONEBEACON 401(K) SAVINGS AND EMPLOYEE
STOCK OWNERSHIP PLAN

To:    Computershare Trust Company, N.A., as Agent:

        As a participant in the above-referenced Plan, I hereby instruct Vanguard Fiduciary Trust Company, the trustee (the "Trustee") of the OneBeacon 401(k) Savings and Employee Stock Ownership Plan, which holds the common shares of White Mountains Insurance Group, Ltd., to withdraw from the Offer (as defined in the Offer to Purchase) all Plan Shares (as defined in the Letter from Vanguard Fiduciary Trust Company to Participants in the OneBeacon 401(k) Savings and Employee Stock Ownership Plan, dated August 17, 2017 (the "Plan Letter")) that I previously instructed the Trustee to tender on my behalf, pursuant to the Offer (as defined in the Letter).

DO NOT USE THIS FORM TO TENDER YOUR PLAN SHARES.

ONLY USE THIS FORM TO WITHDRAW YOUR PREVIOUS TENDER OF
PLAN SHARES FROM THE OFFER.

Mail this withdrawal notice promptly to:

VOLUNTARY CORPORATE ACTION COY: WTM VANGAURD 401(k) PLAN

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QuickLinks

Exhibit (a)(1)(H)

LETTER TO PARTICIPANTS IN THE ONEBEACON 401(K) SAVINGS AND EMPLOYEE STOCK OWNERSHIP PLAN August 17, 2017
QUESTIONS AND ANSWERS WITH RESPECT TO TENDER RIGHTS OF PARTICIPANTS IN THE ONEBEACON 401(K) SAVINGS AND EMPLOYEE STOCK OWNERSHIP PLAN DESCRIPTION OF THE OFFER
OPERATION OF THE PLAN DURING THE OFFER
REINVESTMENT OF OFFER PROCEEDS
ELECTION FORM OneBeacon 401(k) Savings and Employee Stock Ownership Plan (the "Plan") Your election must be received no later than 5:00 p.m., New York City time, on September 12, 2017, to be included in the tabulation, unless the Offer is extended.
Note: The sum of the whole percentages you write in must not exceed 100%. If the sum of all such whole percentages exceeds 100%, none of the Plan Shares allocated to your account will be tendered.
NOTICE OF WITHDRAWAL FROM THE TENDER OFFER WHITE MOUNTAINS INSURANCE GROUP, LTD. TENDER OFFER FOR PARTICIPANTS IN ONEBEACON 401(K) SAVINGS AND EMPLOYEE STOCK OWNERSHIP PLAN
DO NOT USE THIS FORM TO TENDER YOUR PLAN SHARES. ONLY USE THIS FORM TO WITHDRAW YOUR PREVIOUS TENDER OF PLAN SHARES FROM THE OFFER. Mail this withdrawal notice promptly to