SUMMARY

This summary highlights information contained elsewhere in this joint proxy statement/prospectus and may not contain all the information that is important to you with respect to the merger, the stock issuance and the other matters being considered at the Alleghany and Transatlantic special meetings. Alleghany and Transatlantic urge you to read the remainder of this joint proxy statement/prospectus carefully, including the attached Annexes, and the other documents to which we have referred you. See also the section entitled “Where You Can Find More Information.” We have included page references in this summary to direct you to a more complete description of the topics presented below.

The Companies

Alleghany Corporation (See page 36)

Alleghany is a Delaware corporation engaged in the property and casualty and surety insurance business through its wholly owned subsidiary Alleghany Insurance Holdings LLC (which we refer to as “AIHL”). AIHL’s insurance business is conducted through its wholly owned subsidiaries RSUI Group, Inc. (which we refer to as “RSUI”), Capitol Transamerica Corporation and Platte River Insurance Company (which we refer to collectively as “CATA”), and Pacific Compensation Corporation (which we refer to as “PCC”). AIHL Re LLC (which we refer to as “AIHL Re”), a captive reinsurance subsidiary of AIHL, has in the past provided reinsurance to Alleghany operating units and affiliates. Alleghany’s equity investments, including those held by AIHL’s insurance operating units, are managed primarily by Alleghany Capital Partners LLC, an indirect, wholly owned subsidiary of Alleghany. Alleghany also owns and manages properties in the Sacramento, California region through its subsidiary Alleghany Properties Holdings LLC (which we refer to as “Alleghany Properties”). In addition, Alleghany owns approximately 33 percent of the outstanding shares of common stock of Homesite Group Incorporated (which we refer to as “Homesite”), a national, full-service, mono-line provider of homeowners insurance, and approximately 38 percent of ORX Exploration, Inc. (which we refer to as “ORX”), a regional oil and gas exploration and production company. Alleghany also makes strategic investments in operating companies and conducts other activities.

Shares of Alleghany common stock are traded on the NYSE under the symbol “Y”. Following the merger, shares of Alleghany common stock will continue to be traded on the NYSE under the symbol “Y”.

The principal executive offices of Alleghany are located at 7 Times Square Tower, New York, NY 10036 and its telephone number is (212) 752-1356.

Transatlantic Holdings, Inc. (See page 36)

Transatlantic Holdings, Inc. is a holding company incorporated in the State of Delaware. Transatlantic, through its wholly owned subsidiaries, Transatlantic Reinsurance Company^® (which we refer to as “TRC”), Trans Re Zurich Reinsurance Company Ltd., acquired by TRC in 1996 (which we refer to as “TRZ”), and Putnam Reinsurance Company (which we refer to as “Putnam”) (contributed by Transatlantic to TRC in 1995), offers reinsurance capacity for a full range of property and casualty products, directly and through brokers, to insurance and reinsurance companies, in both the domestic and international markets on both a treaty and facultative basis. One or both of TRC and Putnam is licensed, accredited, authorized or can serve as a reinsurer in 50 states and the District of Columbia in the United States and in Puerto Rico and Guam. Through its international locations, Transatlantic has operations worldwide, including Bermuda, Canada, seven locations in Europe, three locations in Central and South America, two locations in Asia (excluding Japan), and one location in each of Japan, Australia and Africa. TRC is licensed in Bermuda, Canada, Japan, the United Kingdom, the Dominican Republic, the Hong Kong Special Administrative Region, the People’s Republic of China and Australia. Transatlantic was originally formed in 1986 under the name PREINCO Holdings, Inc. as a holding

company for Putnam. Transatlantic’s name was changed to Transatlantic Holdings, Inc. on April 18, 1990 following the acquisition on April 17, 1990 of all of the common stock of TRC in exchange for shares of Transatlantic common stock.

Transatlantic’s common stock is traded on the NYSE under the symbol “TRH”. Upon completion of the merger, shares of Transatlantic common stock currently listed on the NYSE will cease to be listed on the NYSE and will subsequently be deregistered under the Exchange Act.

The principal executive offices of Transatlantic are located at 80 Pine Street, New York, New York 10005 and its telephone number is (212) 365-2200.

Shoreline Merger Sub, Inc. (See page 37)

Shoreline Merger Sub, Inc., or Merger Sub, is a wholly owned subsidiary of Alleghany and a Delaware corporation. Merger Sub was originally formed on November 10, 2011 as a Delaware limited liability company called Shoreline Merger Sub, LLC for the sole purpose of effecting the merger. At the request of Transatlantic, Alleghany converted Shoreline Merger Sub, LLC into a Delaware corporation which has been assigned the rights and assumed the obligations of “Merger Sub” under the merger agreement, and as such all references to “Merger Sub” in the merger agreement and in this joint proxy statement/prospectus shall be deemed to refer to such corporation. In the merger, Transatlantic will be merged with and into Merger Sub, with Merger Sub surviving as a wholly owned subsidiary of Alleghany. Upon completion of the merger, Alleghany intends to operate Transatlantic as an independent standalone subsidiary of Alleghany, which will be renamed “Transatlantic Holdings, Inc.”

The principal executive offices of Merger Sub are located at 7 Times Square Tower, New York, NY 10036 and its telephone number is (212) 752-1356.

Risk Factors (See page 27)

Before voting at the Alleghany special meeting or the Transatlantic special meeting, you should carefully consider all of the information contained in or incorporated by reference into this joint proxy statement/prospectus, as well as the specific factors under the heading “Risk Factors,” including the risks that:

The Merger

The Merger Agreement (See page 126)

Alleghany and Transatlantic have entered into the merger agreement attached as Annex A to this joint proxy statement/prospectus. Alleghany and Transatlantic encourage you to read the entire merger agreement carefully because it is the principal document governing the merger and the stock issuance.

Effects of the Merger (See page 54)

Subject to the terms and conditions of the merger agreement, at the effective time of the merger, Transatlantic will be merged with and into Merger Sub, with Merger Sub surviving the merger as a wholly owned subsidiary of Alleghany. Upon completion of the merger, Alleghany intends to operate Transatlantic as an independent standalone subsidiary of Alleghany. We expect that, on a fully diluted basis, the existing stockholders of Alleghany and the former stockholders of Transatlantic will own approximately 51% and 49%, respectively, of the outstanding Alleghany common stock following the merger.

Consideration to be Received in the Merger (See page 127)

If the merger is completed, Transatlantic stockholders will have the right to elect to receive merger consideration for each of their shares of Transatlantic common stock in the form of cash or shares of Alleghany common stock, subject to proration in the circumstances described below. In the event of proration, a Transatlantic stockholder may receive merger consideration in respect of some or all of the Transatlantic shares held by such stockholder in a form other than that which such stockholder elected. If the aggregate consideration to be paid to any holder of Transatlantic common stock would result in such holder receiving a fractional share of Alleghany common stock, cash shall be paid in lieu of such fractional share.

The value of the merger consideration will fluctuate with the market price of the Alleghany common stock and will be determined based on the average five-day Alleghany closing price. As explained in more detail in this document, whether a Transatlantic stockholder makes a cash election, a stock election or no election, the value of the consideration that such stockholder receives as of the date of completion of the merger will be approximately equivalent based on the average five-day Alleghany closing price used to calculate the merger consideration. A Transatlantic stockholder may specify different elections with respect to different shares that such stockholder holds (e.g., if a Transatlantic stockholder owns 100 shares of Transatlantic common stock, that stockholder could make a cash election with respect to 50 shares and a stock election with respect to the other 50 shares).

The aggregate amount of cash to be paid to Transatlantic stockholders is fixed in the merger agreement at $816,007,519. As a result, if the cash election is oversubscribed or undersubscribed, then certain adjustments will be made to the merger consideration to proportionately reduce the cash or stock amounts received by the Transatlantic stockholders in the manner described below in the section entitled “The Merger Agreement—Consideration to be Received in the Merger—Proration.” To the extent that the number of outstanding shares of Transatlantic increases between the date of the merger agreement and the effective time of the merger, due to the vesting of stock-settled awards or as otherwise permitted by the merger agreement, the aggregate number of shares of Alleghany common stock to be issued as consideration in the merger will be increased accordingly, but the aggregate amount of cash to be paid as consideration will not change.

As an example, based on the average of the closing prices of Alleghany common stock for the five trading days ending on November 18, 2011 (the last trading day before announcement of the merger), for each share of Transatlantic common stock held, a Transatlantic stockholder would receive either approximately $59.51 in cash or 0.1905 shares of Alleghany common stock, having a market value of $59.51 based on such average five-day Alleghany closing price, subject to proration if cash was oversubscribed or undersubscribed. As another example, based on the average of the closing prices of Alleghany common stock for the five trading days ending on

December 19, 2011 (the most recent practicable trading date before filing of this joint proxy statement/prospectus), for each share of Transatlantic common stock held, a Transatlantic stockholder would receive either approximately $55.42 in cash or 0.1951 shares of Alleghany common stock, having a market value of $55.42 based on such average five-day Alleghany closing price, subject to proration if cash was oversubscribed or undersubscribed. We will compute the actual amount of cash and number of shares of Alleghany common stock that each Transatlantic stockholder will receive in the merger using the formula contained in the merger agreement. For a summary of the formula contained in the merger agreement, see “The Merger Agreement.”

The following table illustrates the value of the merger consideration for different hypothetical five-day average closing prices of the Alleghany common stock on the NYSE for the five trading days immediately preceding the day on which the merger is completed and, for illustrative purposes only, the effects of proration assuming that 50% of Transatlantic shares elect cash consideration and 50% of Transatlantic shares elect stock consideration. For simplicity, the table assumes that there are 57,386,934 shares of Transatlantic common stock outstanding (which represents the number of shares of Transatlantic common stock outstanding on December 19, 2011, the most recent practicable day before filing of this joint proxy statement/prospectus), that all Transatlantic stockholders make elections and that no Transatlantic stockholders have exercised appraisal rights.

The table above is illustrative only. The value of the merger consideration that a Transatlantic stockholder actually receives will be based on the actual average five-day Alleghany closing price, as described below. The actual average five-day Alleghany closing price may be outside the range of the amounts set forth above, and as a result, the actual value of the merger consideration per share of Transatlantic common stock may not be shown in the above tables.

Election Form (See page 131)

Record holders of shares of Transatlantic common stock on January 4, 2012, the Transatlantic record date, are receiving an election form enclosed with this joint proxy statement/prospectus with instructions for making cash and/or stock elections. Transatlantic stockholders must properly complete and deliver to the exchange agent an election form by the election deadline (which will be announced in a press release by Alleghany and Transatlantic at least five business days prior to such deadline), accompanied by their Transatlantic stock

certificates (or a properly completed notice of guaranteed delivery). The election form also includes delivery instructions with respect to book-entry shares. Transatlantic stockholders should NOT send in their stock certificates with their proxy card. Once Transatlantic stockholders have tendered their Transatlantic stock certificates to the exchange agent, they may not transfer their shares of Transatlantic common stock represented by those stock certificates until the merger is completed, unless they revoke their election by written notice to the exchange agent that is received prior to the election deadline. If the merger is not completed and the merger agreement is terminated, stock certificates will be returned by the exchange agent.

If Transatlantic stockholders fail to submit a properly completed election form, together with their Transatlantic stock certificates (or a properly completed notice of guaranteed delivery), if any, prior to the election deadline, they will be deemed not to have made an election. As non-electing holders, they will be paid merger consideration in an amount per share that is approximately equivalent in value to the amount paid per share to holders making elections, but they may be paid all in cash, all in Alleghany common stock, or in part cash and in part Alleghany common stock, depending on the remaining pool of cash and Alleghany common stock available for paying merger consideration after honoring the cash elections and stock elections that other stockholders have made, and without regard to the preferences of such non-electing holders.

Treatment of Transatlantic Stock Options and Other Long-Term Incentive Awards (See page 119)

Each outstanding stock option to acquire Transatlantic common stock, whether vested or unvested, will be converted into the right to receive a cash payment equal to the value of such stock option based on an amount determined using the Black-Scholes valuation methodology based on assumptions that are agreed upon by Transatlantic and Alleghany.

Each outstanding Transatlantic restricted stock unit (including each performance-based Transatlantic restricted stock unit) held by an employee or former employee of Transatlantic will be converted into a right to receive cash in an amount equal to the per share merger consideration, with the same terms and conditions as were applicable under such restricted stock unit prior to the conversion, with (i) the cash value of the converted Transatlantic restricted stock units held by employees or former employees of Transatlantic who were designated as participants in the Transatlantic Senior Partners Plan or Partners Plan (collectively, the “Partners Plans”) to be deemed to be notionally invested in common units of the surviving company or (ii) the cash value of the converted Transatlantic restricted stock units held by employees or former employees of Transatlantic who were not designated as participants in a Partners Plan may, if so elected by such employee or former employee, be deemed to be notionally invested in common units of the surviving company. For outstanding Transatlantic restricted stock units that are subject to performance goals for which the performance period is not completed as of the closing date of the merger, the level of achievement of the performance goals relating to such performance-based Transatlantic restricted stock units will either be determined based on (i) actual performance as of the closing date of the merger for performance periods that end on or prior to the date that is nine months following the closing date of the merger or (ii) the target level for any performance-based Transatlantic restricted stock unit with a performance period that ends more than nine months following the closing date of the merger.

Each outstanding Transatlantic restricted stock unit held by a non-employee director of Transatlantic will be converted into a fully vested right to receive cash in an amount equal to the per share merger consideration and will be paid in cash at the time specified under the Transatlantic 2008 Non-Employee Directors Stock Plan, with amounts in respect of Transatlantic restricted stock units held by non-employee directors who serve on the Alleghany board of directors following the closing date of the merger to be deemed notionally invested in common stock of Alleghany until the date of distribution to such non-employee director.

Recommendation of the Board of Directors of Alleghany (See page 38)

In reaching its decision to approve the merger agreement and recommend approval by Alleghany stockholders of the stock issuance proposal, the Alleghany board of directors consulted with Alleghany’s

management, as well as with Alleghany’s legal and financial advisors, and also considered a number of factors that the Alleghany board of directors views as supporting its decision, including, but not limited to, the following:

After careful consideration, the Alleghany board of directors approved the merger agreement and the transactions contemplated thereby, including the merger and the stock issuance, and determined that the merger agreement and the transactions contemplated thereby, including the merger and the stock issuance, are advisable and in the best interests of Alleghany and its stockholders. For more information regarding the factors considered by the Alleghany board of directors in reaching its decision to approve the merger agreement and the transactions thereby contemplated, see the section entitled “The Merger—Alleghany’s Reasons for the Merger; Recommendation of the Alleghany Board of Directors.” The Alleghany board of directors recommends that the Alleghany stockholders vote (i) “FOR” the stock issuance proposal and (ii) “FOR” the Alleghany adjournment proposal.

Recommendation of the Board of Directors of Transatlantic (See page 45)

In reaching its decision to approve the merger agreement and recommend adoption of the merger agreement by the Transatlantic stockholders, the Transatlantic board of directors consulted with Transatlantic’s management, as well as with Transatlantic’s legal and financial advisors, and also considered a number of factors that the Transatlantic board of directors views as supporting its decision, including, but not limited to, the following:

After careful consideration, the Transatlantic board of directors unanimously approved the merger agreement and determined that the merger agreement and the transactions contemplated thereby, including the merger, are advisable and in the best interests of Transatlantic and its stockholders. For more information regarding the factors considered by the Transatlantic board of directors in reaching its decision to approve the merger agreement and the merger, see the section entitled “The Merger—Transatlantic’s Reasons for the Merger; Recommendation of the Transatlantic Board of Directors.” The Transatlantic board of directors unanimously recommends that Transatlantic stockholders vote (i) “FOR” the adoption of the merger agreement, (ii) “FOR” the Transatlantic adjournment proposal and (iii) “FOR” the golden parachute proposal.

Opinions of Alleghany’s Financial Advisors (See page 78)

In connection with a meeting of the Alleghany board of directors held to evaluate the proposed merger, each of UBS Securities LLC (which we refer to as “UBS”) and Morgan Stanley & Co. LLC (which we refer to as “Morgan Stanley”) delivered to Alleghany’s board of directors written opinions, dated November 20, 2011, to the effect that, as of that date and based on and subject to various assumptions, matters considered, qualifications and limitations described in their respective opinions, the merger consideration, to be paid by Alleghany in the merger was fair, from a financial point of view, to Alleghany.

The full texts of the written opinions of UBS and Morgan Stanley, each dated November 20, 2011, which set forth, among other things, the assumptions made, procedures followed, matters considered, and limitations, qualifications and conditions to the review undertaken by each of UBS and Morgan Stanley in connection with their opinions, are attached as Annex B and Annex C, respectively, to this joint proxy statement/prospectus and are incorporated herein by reference. Holders of Alleghany common stock are encouraged to read each opinion carefully in its entirety. The opinions were directed to, and provided for the benefit of, the Alleghany board of directors (in its capacity as such), in connection with, and for the purpose of, its evaluation of the merger consideration to be paid by Alleghany in the Transaction, and do not address any other aspect of the merger. Neither opinion addresses the prices at which the Alleghany common stock will trade following consummation of the merger or at any time. Neither opinion addresses the relative merits of the merger as compared to other business strategies or transactions that might be available to Alleghany or Alleghany’s underlying business decision to effect the merger. Neither opinion constitutes a recommendation to any stockholder as to how such stockholder should vote or act with respect to the merger.

Opinion of Transatlantic’s Financial Advisor—Goldman, Sachs & Co. (See page 94)

Goldman, Sachs & Co. (which we refer to as “Goldman Sachs”) delivered its opinion to the board of directors of Transatlantic that, as of November 20, 2011 and based upon and subject to the limitations and assumptions set forth therein, the merger consideration to be paid to the holders (other than Alleghany and its affiliates) of the outstanding shares of Transatlantic common stock pursuant to the merger agreement was fair from a financial point of view to such holders. The merger consideration is subject to certain procedures and limitations contained in the merger agreement, as to which procedures and limitations Goldman Sachs expressed no opinion.

The full text of the written opinion of Goldman Sachs, dated as of November 20, 2011, which sets forth assumptions made, procedures followed, matters considered and limitations on the review undertaken in connection with the opinion, is attached to this joint proxy statement/prospectus as Annex D. Goldman Sachs provided its opinion for the information and assistance of the board of directors of Transatlantic in connection with its consideration of the merger. The Goldman Sachs opinion is not a recommendation as to how any holder of shares of Transatlantic common stock should vote or make any election with respect to the merger or any other matter. Pursuant to an engagement letter between Transatlantic and Goldman Sachs, Transatlantic will pay Goldman Sachs a transaction fee of $21,000,000, which is contingent upon consummation of the merger.

For a more complete description, see “The Merger—Opinion of Transatlantic’s Financial Advisor—Goldman, Sachs & Co.” in this joint proxy statement/prospectus. See also Annex D to this joint proxy statement/prospectus.

Opinion of Transatlantic’s Financial Advisor—Moelis & Company LLC (See page 101)

Moelis & Company LLC (which we refer to as “Moelis”) delivered its opinion to the Transatlantic board of directors that, as of November 20, 2011 and based upon and subject to the conditions and limitations set forth therein, the merger consideration pursuant to the merger agreement was fair, from a financial point of view, to the holders of Transatlantic common stock.

The full text of the written opinion of Moelis, dated November 20, 2011, which sets forth the assumptions made, procedures followed, matters considered and limitations on the review undertaken in connection with its opinion, is attached to this joint proxy statement/prospectus as Annex E. The summary of Moelis’ opinion contained in this joint proxy statement/prospectus is qualified in its entirety by reference to the full text of the opinion. Moelis provided its opinion for the information and assistance of

the Transatlantic board of directors in connection with its consideration of the merger. Moelis’ opinion does not constitute a recommendation to any holder of Transatlantic common stock as to how such stockholder should vote with respect to the merger or as to which election to make with respect to the merger consideration or any other matter. In addition, Moelis was not requested to opine as to, and its opinion does not in any manner address, Transatlantic’s underlying business decision to effect the merger or the relative merits of the merger as compared to any alternative business strategies or transactions that might be available to Transatlantic. See also “The Merger—Opinion of Transatlantic’s Financial Advisor—Moelis & Company LLC.”

Interests of Alleghany’s Directors and Executive Officers in the Merger (See page 112)

Executive officers of Alleghany have interests in the merger that may be different from, or in addition to, the interests of Alleghany stockholders generally. The Alleghany board of directors was aware of these interests and considered them, among other matters, in approving the merger agreement and the transactions contemplated by the merger agreement, including the merger and the stock issuance, and in recommending that Alleghany stockholders vote to approve the stock issuance and Alleghany adjournment. For additional information regarding the interests of Alleghany directors and executive officers in the merger, please see the section entitled “The Merger—Interests of Alleghany’s Directors and Executive Officers in the Merger.”

Interests of Transatlantic’s Directors and Executive Officers in the Merger (See page 112)

Executive officers and members of the Transatlantic board of directors have interests in the merger that may be in addition to, or different from, the interests of Transatlantic stockholders generally.

As detailed below under “The Merger—Alleghany Board of Directors and Management Following the Merger,” certain of Transatlantic’s executive officers and members of the Transatlantic board of directors will continue to serve as officers or directors of the combined company or Transatlantic (as a subsidiary of Alleghany) upon completion of the merger. Specifically, upon completion of the merger, the Alleghany board of directors will be expanded to 14 members and will include three directors who currently serve on the Transatlantic board of directors. In addition, Mr. Michael C. Sapnar will be appointed as President and Chief Executive Officer of Transatlantic following the merger.

Transatlantic has various equity programs that provide for “double trigger” payments (i.e., payments upon certain termination events in proximity to a “change in control”). The merger will constitute a “change in control” for purposes of such arrangements. Transatlantic, however, does not currently expect the employment of Transatlantic’s executive officers to be terminated at or following the closing of the merger. As such, double trigger vesting under Transatlantic’s equity programs are not expected to be triggered with respect to any executive officers. Further, pursuant to the merger agreement, Transatlantic and Alleghany will each use their respective reasonable best efforts to agree to terms of retention agreements for certain executives, including each of Transatlantic’s executive officers other than Robert F. Orlich.

Each outstanding option to acquire Transatlantic common stock held by an executive officer of Transatlantic, whether vested or unvested, will be converted into the right to receive a cash payment equal to the value of such stock option based on an amount determined using the Black-Scholes valuation methodology based on assumptions that are agreed upon by Transatlantic and Alleghany. Each outstanding restricted stock unit held by an executive officer of Transatlantic (including each performance-based Transatlantic restricted stock unit) will be converted into a right to receive cash in an amount equal to the per share merger consideration, with the terms and conditions as were applicable under such restricted stock unit prior to the conversion (including vesting or forfeiture provisions), with the cash value of the converted Transatlantic restricted stock units deemed to be notionally invested in the common units of the surviving company. Outstanding Transatlantic restricted

stock units held by an executive officer of Transatlantic that are subject to performance goals will be treated as follows: (i) the level of achievement of the applicable performance goal for any performance based Transatlantic restricted stock unit with a performance period that ends on or prior to the date that is nine months following the closing date of the merger will be determined based on actual performance through the closing date of the merger and (ii) the level of achievement of the applicable performance goal for any performance-based Transatlantic restricted stock unit with a performance period that ends more than nine months following the closing date of the merger will be deemed to be earned at target level. Each outstanding Transatlantic restricted stock unit held by a non-employee director of Transatlantic will be converted into a fully vested right to receive cash in an amount equal to the per share merger consideration and will be paid in cash at the time specified under the Transatlantic 2008 Non-Employee Directors Stock Plan, with amounts in respect of Transatlantic restricted stock units held by non-employee directors who continue service with the Alleghany board of directors following the closing date of the merger to be deemed notionally invested in common stock of Alleghany until the date of distribution to such non-employee director. For additional information regarding the interests of Transatlantic directors and executive officers in the merger, please see the section entitled “The Merger—Interests of Transatlantic’s Directors and Executive Officers in the Merger.”

The Transatlantic board of directors was aware of and considered these potential interests, among other matters, in evaluating and negotiating the merger agreement and in recommending that Transatlantic stockholders adopt the merger agreement.

Governance Matters Following the Merger (See page 140)

Alleghany has agreed to take all necessary action to cause, effective at the effective time of the merger, the number of directors on the Alleghany board of directors to be increased from 11 to 14 and three persons who were members of the Transatlantic board of directors immediately prior to the effective time of the merger as mutually agreed by Alleghany and Transatlantic to be elected to the Alleghany board of directors. The Alleghany board of directors has three classes of directors, with one of such classes elected each year. One of the three Transatlantic directors shall become a Class I director, another a Class II director and the third a Class III director.

In addition, Alleghany and Transatlantic have agreed to cause the board of directors of Alleghany after the merger to adopt a written consent (i) appointing Robert F. Orlich as a senior advisor to Transatlantic, (ii) appointing Michael C. Sapnar as President and Chief Executive Officer of Transatlantic and (iii) appointing Weston M. Hicks, Roger B. Gorham, Michael C. Sapnar, Robert F. Orlich and Joseph P. Brandon to the board of directors of Transatlantic Holdings, Inc. following the merger. In addition, following completion of the merger, Joseph P. Brandon, former chief executive of Berkshire Hathaway’s wholly owned subsidiary General Re Corporation, will serve as President of AIHL, Executive Vice President of Alleghany, and Chairman of the board of directors of Transatlantic Holdings, Inc.

Transatlantic has also agreed, prior to the effective time of the merger, to cause all directors of Transatlantic to resign effective as of the effective time of the merger.

In the event that the merger is not completed, the foregoing director elections, officer appointments and director resignations will not take effect.

Regulatory Clearances Required for the Merger (See page 118)

Alleghany and Transatlantic have each agreed to take certain actions in order to obtain regulatory clearance required to consummate the merger. Regulatory clearance required to complete the merger includes expiration or termination of the required waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, and the rules and regulations promulgated thereunder (which we refer to as the “HSR Act”), following

required notifications and review by the Antitrust Division of the U.S. Department of Justice (which we refer to as the “Antitrust Division”) or the Federal Trade Commission (which we refer to as the “FTC”). The parties filed the required notifications with the Antitrust Division and the FTC on December 8, 2011, and the FTC granted early termination of the applicable waiting period on December 16, 2011. Alleghany and Transatlantic have also filed a notification with the relevant competition authorities in Italy and will be filing a notification promptly in Turkey.

In addition, certain insurance regulatory filings will also be required to be made in connection with the merger. State insurance laws in the United States generally require that, prior to the acquisition of an insurance company, the acquiring party must obtain approval from the insurance commissioner of the insurance company’s state of domicile, and the parties have and will make the required filings in accordance with such laws. In addition, applications or notifications have been or will be filed with various insurance regulatory authorities outside of the United States in connection with the changes in control that may be deemed to occur as a result of the transactions contemplated by the merger agreement. Receipt of approval by the New York DFS is a condition to completion of the merger. It is also a condition to completion of the merger that all other consents of or filings with insurance regulators shall have been obtained or made except where the failure to obtain such consents or make such filings would not reasonably be expected to be materially adverse to Alleghany and its subsidiaries, taken as a whole, or Transatlantic and its subsidiaries, taken as a whole (after giving effect to the merger).

While Alleghany and Transatlantic expect to obtain all required regulatory clearances, we cannot assure you that these regulatory clearances will be obtained or that the granting of these regulatory clearances will not involve the imposition of additional conditions on the completion of the merger, including the requirement to divest assets, or require changes to the terms of the merger agreement. These conditions or changes could result in the conditions to the merger not being satisfied.

Effective Time and Completion of the Merger (See page 127)

Alleghany and Transatlantic hope to complete the merger as soon as reasonably practicable and expect the closing of the merger to occur in the first quarter of 2012. However, the merger is subject to various regulatory clearances and the satisfaction or waiver of other conditions, and it is possible that factors outside the control of Alleghany and Transatlantic could result in the merger being completed at an earlier time, a later time or not at all. There may be a substantial amount of time between the dates on which the special meetings are held and the date on which the merger is completed.

Conditions to Completion of the Merger (See page 141)

The obligations of Alleghany and Transatlantic to complete the merger are subject to the satisfaction of the following conditions:

In addition, each of Alleghany’s and Transatlantic’s obligations to effect the merger is subject to the satisfaction or waiver of the following additional conditions:

Transatlantic’s obligation to effect the merger is also subject to the Alleghany board of directors having taken the actions described in the section entitled “The Merger Agreement—Governance Matters Following the Merger.” See the section entitled “The Merger Agreement—Conditions to Completion of the Merger” for a further discussion of the conditions to closing of the merger.

No Solicitation of Alternative Proposals (See page 137)

The merger agreement precludes Alleghany and Transatlantic from soliciting or engaging in discussions or negotiations with a third party with respect to a proposal for a competing transaction, including the acquisition of a significant interest in Alleghany’s or Transatlantic’s common stock or assets. However, if Alleghany or Transatlantic receives an unsolicited proposal from a third party for a competing transaction that Alleghany’s or Transatlantic’s board of directors, as applicable, among other things, determines in good faith (after consultation

with its outside legal advisors and financial advisors) (i) constitutes or is reasonably likely to lead to a proposal that is superior to the merger and (ii) with respect to which the failure to enter into discussions would result in a breach of its fiduciary duties under applicable law, Alleghany or Transatlantic, as applicable, may, subject to certain conditions, furnish non-public information to and enter into discussions with, and only with, that third party regarding such competing transaction.

See the section entitled “The Merger Agreement—No Solicitation of Alternative Proposals” for a further discussion of each party’s covenant not to solicit alternative acquisition proposals.

Termination of the Merger Agreement (See page 142)

Generally, the merger agreement may be terminated and the merger may be abandoned at any time prior to the completion of the merger (except as specified below, including after the required Alleghany stockholder approvals or Transatlantic stockholder approvals are obtained):

See the section entitled “The Merger Agreement—Termination of the Merger Agreement” for a further discussion of the rights of each of Alleghany and Transatlantic to terminate the merger agreement.

Expenses and Termination Fees; Liability for Breach (See page 143)

Generally, all fees and expenses incurred in connection with the merger agreement and the transactions contemplated by the merger agreement will be paid by the party incurring those expenses, subject to the specific exceptions discussed in this joint proxy statement/prospectus whereby Alleghany or Transatlantic, as the case may be, may be required to pay a termination fee of $115 million or $35 million and/or the reimbursement of expenses up to a maximum amount of $35 million.

See the section entitled “The Merger Agreement—Expenses and Termination Fees; Liability for Breach” for a further discussion of the circumstances under which such termination fees and/or expense reimbursement will be required to be paid.

Accounting Treatment (See page 152)

Alleghany and Transatlantic each prepare its financial statements in accordance with accounting principles generally accepted in the United States of America (which we refer to as “GAAP”) and any statutory accounting principles prescribed or permitted by the domiciliary state insurance department of the applicable subsidiary (which we refer to as “SAP”). The merger will be accounted for using the acquisition method of accounting. Alleghany will be treated as the acquirer for accounting purposes.

See the section entitled “Accounting Treatment” for a further discussion of the accounting treatment of the transaction.

Appraisal Rights (See page 120)

Depending upon the elections made by holders of shares of Transatlantic common stock with respect to the form of consideration to be received in the merger, Delaware law may entitle the holders of shares of Transatlantic common stock, who comply with the procedures specified in Section 262, to have their shares appraised by the Delaware Court of Chancery. Specifically, holders, if any, of shares of Transatlantic common stock who make a stock election (as more fully explained below under “The Merger Agreement—Consideration To Be Received in the Merger—Stock Election”), but are forced to accept cash consideration in respect of such shares by reason of proration (and not simply cash in lieu of fractional shares) (as more fully explained below under “The Merger Agreement—Consideration To Be Received in the Merger—Proration”), would be entitled to have the “fair value” of such shares appraised by the Delaware Court of Chancery if they otherwise comply with the procedures set forth in Section 262. Under Delaware law, holders of shares of Transatlantic common stock who choose not to make an election with respect to the form of merger consideration to be received for their shares will not be entitled to appraisal rights.

As of the date of the mailing of this joint proxy statement/prospectus, we cannot definitively state whether appraisal rights will be available as a result of the merger because (i) the availability of appraisal rights depends on whether the cash consideration is undersubscribed to the extent that appraisal rights would be available, and (ii) we will not know whether the cash consideration is undersubscribed to the extent that appraisal rights would be available until the election deadline (as more fully explained below under “The Merger Agreement—Consideration To Be Received in the Merger”). In the event that the cash consideration is undersubscribed to the extent that appraisal rights would be available, Transatlantic stockholders who have otherwise complied with the requirements of Section 262 will be advised of the availability of appraisal rights within ten days of the merger in the notice of the effective date of the merger required by Section 262. If the cash consideration is not undersubscribed to the extent that appraisal rights would be available, Alleghany will issue a public announcement and file a Current Report on Form 8-K with the SEC informing Transatlantic stockholders that appraisal rights will not be available in connection with the merger.

Because Transatlantic stockholders may be entitled to appraisal rights under certain circumstances, we urge you to read the summary of appraisal rights contained in this joint proxy statement/prospectus under the section

entitled “The Merger—Appraisal Rights” as well as Section 262, which is attached hereto as Annex G. If you wish to preserve the ability to exercise appraisal rights, you must make a written demand for appraisal of your shares as described in the section entitled “The Merger—Appraisal Rights” and in DGCL Section 262.

Litigation Related to the Merger (See page 124)

On November 22, 2011, a putative stockholder class action lawsuit was filed against Transatlantic, Transatlantic’s directors, Alleghany, and Shoreline Merger Sub, LLC in New York State court in connection with the merger agreement: Clark v. Transatlantic Holdings, et al., Index No. 653256/2011 (Supreme Court of the State of New York, County of New York). The lawsuit asserts that the members of the Transatlantic board of directors breached a fiduciary duty in connection with the approval of the merger and that Transatlantic, Alleghany and Shoreline Merger Sub, LLC aided and abetted the alleged breaches of fiduciary duty. Transatlantic, Alleghany and their respective directors believe this lawsuit is without merit and intend to defend it vigorously. In addition, Transatlantic is party to a number of lawsuits relating to (i) the since-terminated Agreement and Plan of Merger, dated as of June 12, 2011, by and among Allied World, GO Sub, LLC and Transatlantic and (ii) the terminated exchange offer and solicitation of written consents commenced by Validus Holdings, Ltd. (which we refer to as “Validus”). For further information regarding the litigation related to the merger, see the section entitled “The Merger—Litigation Related to the Merger.”

Listing of Alleghany Shares; De-listing and Deregistration of Shares of Transatlantic Common Stock (See page 120)

It is a condition to the completion of the merger that the shares of Alleghany common stock to be issued to Transatlantic stockholders pursuant to the merger be authorized for listing on the NYSE at the effective time of the merger, subject to official notice of issuance. Upon completion of the merger, shares of Transatlantic common stock currently listed on the NYSE will cease to be listed on the NYSE and will subsequently be deregistered under the Exchange Act.

See the sections entitled “The Merger—Listing of Alleghany Shares” and “The Merger—De-listing and Deregistration of Transatlantic Common Stock” for a further discussion of the listing of Alleghany shares and de-listing of Transatlantic common stock in connection with the merger.

The Merger Will Generally Be Tax-Free to Holders of Transatlantic Common Stock That Receive Only Alleghany Common Stock and Taxable to Holders That Receive Cash (See page 149)

Neither Alleghany nor Transatlantic will be required to complete the merger unless it receives a legal opinion to the effect that the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code. These opinion conditions will not be waivable by a party after such party’s stockholders have approved the stock issuance proposal (in the case of Alleghany) or the proposal to adopt the merger agreement (in the case of Transatlantic), unless further approval of the stockholders of Alleghany or Transatlantic, as applicable, is obtained with appropriate disclosure. Accordingly, we expect the transaction to generally be tax-free to holders of Transatlantic common stock for United States federal income tax purposes to the extent that such holders receive only shares of Alleghany common stock pursuant to the merger. Those holders receiving solely cash for their Transatlantic common stock will generally recognize gain or loss equal to the difference between the amount of cash received and their tax basis in their shares of Transatlantic common stock. Those holders receiving both Alleghany common stock and cash for their Transatlantic common stock will generally recognize gain, but not loss, equal to the lesser of (1) the amount of cash received and (2) the excess of the “amount realized” in the transaction (i.e., the fair market value of the Alleghany common stock at the effective time of the merger plus the amount of cash received) over their tax basis in their Transatlantic common stock. In certain circumstances, such gain or, in the case of recipients of cash only, the entire amount of cash received, could be

taxable as a dividend rather than capital gain. For a further summary of the United States federal income tax consequences of the merger to holders of Transatlantic common stock, please see “Material U.S. Federal Income Tax Consequences.”

The U.S. federal income tax consequences described above may not apply to all holders of Transatlantic common stock. Your tax consequences will depend on your individual situation. Accordingly, we strongly urge you to consult your tax advisor for a full understanding of the particular tax consequences of the merger to you.

The Meetings

The Alleghany Special Meeting (See page 38)

The Alleghany special meeting will be held at the Harvard Club of New York City, 35 West 44^th Street, New York, New York, on February 6, 2012, at 10:00 a.m., New York City time, to consider and vote upon the following matters:

Completion of the merger is conditioned on, among other things, approval of the stock issuance proposal.

The approval of the stock issuance proposal requires the affirmative vote of holders of a majority of the shares of Alleghany common stock, present in person or represented by proxy, at the Alleghany special meeting and entitled to vote on the proposal, assuming a quorum is present. Approval of the Alleghany adjournment proposal requires the affirmative vote of the holders of a majority of the shares of Alleghany common stock entitled to vote and present in person or represented by proxy, whether or not a quorum is present.

Only holders of record of Alleghany common stock at the close of business on January 4, 2012, the Alleghany record date, are entitled to notice of, and to vote at, the Alleghany special meeting or any adjournments or postponements thereof. At the close of business on the Alleghany record date, [—] shares of Alleghany common stock were issued and outstanding, approximately [—]% of which were held by Alleghany’s directors and executive officers and their affiliates. We currently expect that Alleghany’s directors and executive officers will vote their shares in favor of each of the proposals to be considered and voted upon at the Alleghany special meeting, although, except as described below, no director or executive officer has entered into any agreement obligating him or her to do so. Certain members of the Kirby family (including Jefferson W. Kirby, the Chairman of the Alleghany board of directors, in his capacity as an Alleghany stockholder) with longstanding ownership interests in Alleghany common stock have entered into voting agreements with Transatlantic. The voting agreements provide, among other things, that these Alleghany stockholders have irrevocably agreed, on the terms and subject to the conditions specified in the voting agreements, to vote all shares of Alleghany common stock owned by such stockholders in favor of the stock issuance proposal, against competing proposals and against any action or agreement that would be expected to materially impair the ability of Alleghany or Merger Sub to complete the merger. A form of voting agreement entered into by these Alleghany stockholders is included as Annex F hereto. As of November 18, 2011, the last trading day before announcement of the merger, these stockholders held an aggregate of approximately 1,594,958 shares of Alleghany common stock (representing approximately 18.65% of the outstanding shares of Alleghany common stock as of November 18, 2011, and [—]% of the outstanding shares of Alleghany common stock as of the Alleghany record date).

Alleghany may postpone or adjourn its special meeting to a date that is no later than 30 days after the date on which the original special meeting was scheduled to be held (i) with the consent of Transatlantic, (ii) in order for a quorum to be present, (iii) to allow reasonable additional time for the filing and mailing of any supplemental disclosure which must be disseminated under applicable law, (iv) to allow reasonable additional time to solicit additional proxies, (v) if required by applicable law, or (vi) if Alleghany intends to make an adverse recommendation change.

The Alleghany board of directors has approved the merger agreement and determined that the merger agreement and the transactions contemplated thereby, including the merger and the stock issuance, are advisable and in the best interests of Alleghany and its stockholders. The Alleghany board of directors recommends that Alleghany stockholders vote “FOR” the stock issuance and “FOR” the Alleghany adjournment proposal. See “The Alleghany Special Meeting” for further discussion of the Alleghany special meeting.

The Transatlantic Special Meeting (See page 45)

The Transatlantic special meeting will be held at The Down Town Association, 60 Pine Street, New York, New York, on February 6, 2012, at 10:00 a.m., New York City time, to consider and vote upon the following matters:

Completion of the merger is conditioned on, among other things, adoption of the merger agreement.

You may cast one vote for each share of Transatlantic common stock you own. However, to satisfy the requirements of the New York DFS, on June 8, 2009, Davis Advisors, entered into an agreement with Transatlantic whereby Davis Advisors agreed to vote the number of shares of Transatlantic common stock owned by Davis Advisors in excess of 9.9% of Transatlantic’s outstanding shares in a manner proportionate to the vote of the owners of the shares (excluding Davis Advisors, stockholders beneficially owning more than 10% of Transatlantic’s outstanding shares, and directors and officers of Transatlantic) voting on such matters. On November 21, 2011, Davis Advisors publicly stated its current intention to vote in support of the merger but reserves the right to change its mind. As of November 21, 2011, Davis Advisors was the beneficial holder of approximately 14,278,940 shares of Transatlantic common stock (representing approximately 24.9% of the outstanding shares of Transatlantic common stock as of November 21, 2011 and [—]% of the outstanding shares of Transatlantic common stock as of the Transatlantic record date).

Adoption of the merger agreement requires the affirmative vote of the holders of a majority of the outstanding shares of Transatlantic common stock entitled to vote thereon. Approval of the Transatlantic adjournment proposal requires the affirmative vote of the holders of a majority of the shares of Transatlantic common stock entitled to vote and present in person or represented by proxy, whether or not a quorum is present. Approval of the golden parachute proposal requires the affirmative vote of holders of a majority of the shares of Transatlantic common stock present in person or represented by proxy at the Transatlantic special meeting and entitled to vote thereon, assuming a quorum is present. Neither approval of the Transatlantic adjournment proposal nor the golden parachute proposal is a condition to completion of the merger.

Only holders of record of Transatlantic common stock at the close of business on January 4, 2012, the Transatlantic record date, are entitled to notice of, and to vote at, the Transatlantic special meeting or any adjournments or postponements thereof. At the close of business on the Transatlantic record date, [—] shares of Transatlantic common stock were issued and outstanding, approximately [—]% of which were held by Transatlantic’s directors and executive officers and their affiliates. We currently expect that Transatlantic’s directors and executive officers will vote their shares in favor of each of the proposals to be considered and voted upon at the Transatlantic special meeting, although no director or executive officer has entered into any agreement obligating him or her to do so.

Transatlantic may postpone or adjourn its special meeting to a date that is no later than 30 days after the date on which the original special meeting was scheduled to be held (i) with the consent of Alleghany, (ii) in order for a quorum to be present, (iii) to allow reasonable additional time for the filing and mailing of any supplemental

disclosure which must be disseminated under applicable law, (iv) to allow reasonable additional time to solicit additional proxies, (v) if required by applicable law, or (vi) if Transatlantic intends to make an adverse recommendation change.

The Transatlantic board of directors has unanimously approved the merger agreement and determined that the merger agreement and the transactions contemplated thereby, including the merger, are advisable and in the best interests of Transatlantic and its stockholders. The Transatlantic board of directors unanimously recommends that Transatlantic stockholders vote “FOR” the adoption of the merger agreement, “FOR” the Transatlantic adjournment proposal and “FOR” the golden parachute proposal. See “The Transatlantic Special Meeting” for further discussion of the Transatlantic special meeting.

Comparison of Stockholders’ Rights (See page 167)

Transatlantic stockholders, whose rights are currently governed by the Transatlantic restated certificate of incorporation (which we refer to as the “Transatlantic charter”) and the Transatlantic amended and restated by-laws (which we refer to as the “Transatlantic bylaws”) will, to the extent such holders receive Alleghany common stock in the merger, upon completion of the merger, become stockholders of Alleghany and their rights will be governed by the restated certificate of incorporation of Alleghany (which we refer to as the “Alleghany charter”) and the amended and restated by-laws of Alleghany (which we refer to as the “Alleghany bylaws”). These differences are described in detail under “Comparison of Stockholders’ Rights.”

Dividends (See page 120)

Under the terms of the merger agreement, each of Alleghany and Transatlantic is prohibited from paying dividends on its common stock and from repurchasing shares of its common stock during the pendency of the merger. However, Transatlantic is permitted to pay to its common stockholders of record on November 16, 2011 the $0.22 per share dividend previously declared, which dividend was paid on December 2, 2011.

Comparative Per Share Market Price Information

The following table presents the closing prices of Alleghany common stock and Transatlantic common stock on the NYSE on November 18, 2011, the last trading day before announcement of the merger, and December 19, 2011, the most recent practicable date prior to the date of this joint proxy statement/prospectus. The table also presents the closing sales prices calculated by averaging the closing sales prices for shares of Alleghany common stock on each of the trading days during the period of five trading days ending on such dates. The table also presents the approximately equivalent value of the per share merger consideration of Transatlantic common stock on those dates, calculated by multiplying the average five-day closing price of Alleghany common stock ending on those dates by 0.145 and adding $14.22, representing the approximate value that Transatlantic stockholders will be entitled to receive, in exchange for each share of Transatlantic common stock they hold at the effective time of the merger, assuming no proration.

The market prices of shares of Alleghany and Transatlantic common stock fluctuate, and the value of the merger consideration will fluctuate with the market price of the Alleghany common stock and will be determined based on the average five-day Alleghany closing price. As a result, we urge you to obtain current market quotations of Alleghany and Transatlantic common stock.

Summary Consolidated Historical Financial Data of Alleghany

The following table sets forth selected historical consolidated financial data of Alleghany. This data is derived from Alleghany’s Consolidated Financial Statements as of and for the five years ended December 31, 2010, 2009, 2008, 2007 and 2006, respectively, and the unaudited quarterly financial statements as of and for the nine months ended September 30, 2011 and 2010, which in the opinion of management include all adjustments necessary for a fair statement of the results for the unaudited interim periods. This selected financial data should be read in conjunction with Alleghany’s Consolidated Financial Statements and related notes included elsewhere in Alleghany’s Annual Report on Form 10-K for the fiscal year ended December 31, 2010, and Alleghany’s quarterly report on Form 10-Q for the quarter ended September 30, 2011, each of which is incorporated by reference in this joint proxy statement/prospectus. See “Where You Can Find More Information.”

Summary Consolidated Historical Financial Data of Transatlantic

The following table sets forth selected historical consolidated financial data of Transatlantic. This data is derived from Transatlantic’s Consolidated Financial Statements as of and for the five years ended December 31, 2010, 2009, 2008, 2007 and 2006, respectively, and the unaudited quarterly financial statements as of and for the nine months ended September 30, 2011 and 2010, which in the opinion of management include all adjustments necessary for a fair statement of the results for the unaudited interim periods. This selected financial data should be read in conjunction with Transatlantic’s Consolidated Financial Statements and related notes included elsewhere in Transatlantic’s Annual Report on Form 10-K for the fiscal year ended December 31, 2010 and Transatlantic’s quarterly report on Form 10-Q for the quarter ended September 30, 2011, each of which is incorporated by reference in this joint proxy statement/prospectus. See “Where You Can Find More Information.”