Document:

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                                                                    Exhibit 4.13

September 26, 2001

[Employee Name
 & Address]

Dear _____________:

As you know, United Defense has achieved an enviable record of achievement since
its acquisition by The Carlyle Group in October 1997. We have delivered critical
developmental successes for our government in major programs such as the Bradley
A3 and the Crusader; we repaid much of the 1997 acquisition debt years ahead of
schedule; and the profits and cash generated by our business have consistently
exceeded our projections.

At the same time, our Company faces new and difficult challenges. The corporate
refinancing which we accomplished in August has created a substantial new debt
burden. In the new Administration and elsewhere, influential policy makers are
calling for the redirection of defense expenditures into areas in which UDLP has
not participated, such as missile defense and homeland defense. We have every
reason to expect that this new environment will require the continued retention,
dedication, and creativity of our key personnel, if United Defense is to
continue its record of success.

Accordingly, the Board of Directors has adopted a new retention incentive
program for key United Defense personnel. I am pleased to inform you that you
have been selected to participate in this program, which will operate as
follows:

               (1) As an inducement for your continuing as a full-time, key
          employee of United Defense (by which we mean United Defense, L.P. and
          any parent, subsidiary, or affiliate entity for which you may work)
          until a Corporate Transaction (as defined below) is concluded, and in
          consideration for your so doing, United Defense hereby promises to pay
          you a special retention bonus (the "Bonus") in the amount of
          __________ dollars ($________), less applicable tax withholdings. As
          used in this commitment letter, a Corporate Transaction means the
          acquisition (i.e., whether by merger, or by stock or asset purchase)
          of all or substantially all of United Defense (whether accomplished at
          the parent level, United Defense Industries, Inc., or at the operating
          company level, United Defense, L.P.) by a corporate or other entity
          not theretofore affiliated with United Defense or its shareholders.

               (2) The Bonus will be paid as and when the owners of United
          Defense are paid in any Corporate Transaction. Thus, for example, in a
          transaction where the entire purchase price is paid on the
          transaction's closing date, the entire

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[Employee Name]
September 26, 2001
Page 2

         Bonus would be paid out at that time. By contrast, if the terms of the
         transaction were to provide that the purchase price would be paid 75%
         on the closing date and 25% one year later, the Bonus would be paid in
         like proportions on those two dates. Also, the Bonus would be fully
         payable regardless of any contingent or so-called earnout provisions in
         the Corporate Transaction. Finally, the Bonus would be paid in whatever
         form - cash, securities, or a mixture of the two - UDLP's owners
         received payment in the Corporate Transaction, except that the Bonus
         payment would in any event include a cash component to assist in paying
         applicable taxes.

The retention incentive undertaken by this letter is in addition to, and does
not replace or in any way reduce, any other UDLP compensation, bonus, stock or
option program, or other compensation arrangement in which you may already
participate. Also, please note that this letter does not constitute an agreement
or guarantee of continued employment, or employment in any particular position
or capacity. It does, however, express the company's firm commitment to pay the
Bonus so long as you remain continuously employed by United Defense until the
completion of a Corporate Transaction.

Please accept my congratulations on your inclusion in this special and important
program. We look forward to United Defense's best years which, with your help,
continue to lie ahead.

                                            Sincerely,

                                            Thomas W. Rabaut
                                            President and Chief
                                            Executive Officer<PAGE>

                                                                    Exhibit 4.14

Severance Pay Plan          Effective Date

                            All of the information in this section is effective
                            as of January 1, 1995 Severance pay is a payment to
                            full-time and regular part-time salaried and
                            non-union hourly employees whose employment with
                            United Defense is permanently terminated for reasons
                            beyond their control and who do not fall into any of
                            the exclusions noted below. Such payments are
                            designed and intended solely to cushion the impact
                            of possible unemployment and to effect orderly
                            business operations until employees' services are no
                            longer required by United Defense.

                            Benefit Amount

                            Severance pay is calculated on a graduated service
                            basis up to two weeks of pay for each year and full
                            month(s) of United Defense service, with a minimum
                            payment of four weeks and a maximum of 52 weeks.

                            ===================== =====================
                            Years of Service      Weeks of Severance
                                                  Per Year of Service
                            ===================== =====================
                            0 but less than 5     1.0 (4 week minimum)
                            5 but less than 11    1.2
                            11 but less than 16   1.4
                            16 but less than 21   1.6
                            21 but less than 26   1.8
                            26 or more            2.0 (52 week maximum)
                            ===================== =====================

                            The following is an example of how the benefit would
                            be calculated for an employee with 16 years and 3
                            months of service.

                            i.e. 16.25 years x 1.6 weeks = 26.0 weeks

                            In computing years of service for severance pay,
                            United Defense service that is recognized as
                            credited service for retirement plan purposes is
                            counted, but no service will be credited under the
                            plan for any period of employment for which the
                            employee previously received a severance payment.

                            All severance pay calculations will be based on
                            current base salary without consideration for any
                            bonus, commissions, incentives, awards, overtime,
                            shift differential or other miscellaneous pay items.
                            Severance pay will always be paid in the calendar
                            year in which it was earned.

                            Exclusions

                            Severance pay will not be paid if the employee:

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                      .     Voluntarily quits, is terminated for cause (which
                            includes poor performance) or dies.

                      .     Is placed on temporary layoff for lack of work.

                      .     Is offered continuing employment by a purchaser
                            (including a partnership or joint venture) of any of
                            United Defense, L.P.'s assets, product lines,
                            functions, facilities or services, or a contractor
                            of services formerly provided internally by United
                            Defense. This exclusion applies whether or not the
                            employee chooses to accept the offer; however, it
                            will not apply if the employment offer requires
                            relocation.

                      .     Refuses the offer of a similar job within United
                            Defense.

                      .     Leaves prior to the last day on which services are
                            required by United Defense

                      .     Elects to voluntarily retire from United Defense
                            under any regular or special retirement program.

                      .     Receives an equivalent benefit as provided by any
                            law.

                    FMC has discretionary authority to construe and interpret
                    the terms of this plan, including, but not limited to,
                    deciding all questions of eligibility and determining the
                    amount, manner, and time of payment of benefits under this
                    plan.

                                       2<PAGE>

                                                                    Exhibit 4.15

                              AMENDMENT AND WAIVER
                              --------------------

          AMENDMENT AND WAIVER (this "Amendment"), dated as of October 15, 2001,
among IRON HORSE INVESTORS, L.L.C., a Delaware limited liability company
("Holdings"), UNITED DEFENSE INDUSTRIES, INC., a Delaware corporation (the
"Borrower"), the lenders from time to time party to the Credit Agreement
referred to below (the "Lenders"), CITICORP USA INC., THE BANK OF NOVA SCOTIA
and CREDIT LYONNAIS NEW YORK BRANCH, as Documentation Agents (the "Documentation
Agents"), LEHMAN COMMERCIAL PAPER INC. as Syndication Agent (the "Syndication
Agent") and BANKERS TRUST COMPANY, as Administrative Agent (the "Administrative
Agent"). Unless otherwise defined herein, all capitalized terms used herein
shall have the respective meanings provided such terms in the Credit Agreement
referred to below.

                              W I T N E S S E T H:
                              - - - - - - - - - -

          WHEREAS, Holdings, the Borrower, the Lenders, the Documentation
Agents, the Syndication Agent and the Administrative Agent are parties to a
Credit Agreement, dated as of August 15, 2001 (as in effect on the date hereof,
the "Credit Agreement");

          WHEREAS, the Borrower intends to effect an initial public offering of
its common stock (the "IPO");

          WHEREAS, Holdings plans to concurrently effect a secondary offering of
a portion of the common stock of the Borrower owned by Holdings (the "Secondary
Offering"); and

          WHEREAS, to permit and give effect to the IPO and Secondary Offering
the parties hereto wish to amend the Credit Documents as herein provided;

          NOW, THEREFORE, it is agreed:

          1. Section 4.02(A) of the Credit Agreement is hereby amended by:

          (a) Changing each reference to "Holdings" in Sections 4.02(A)(c),
4.02(A)(d) and (other than the first reference therein to Holdings) 4.02(A)(e)
to read "the Borrower"; and

          (b) Changing the reference to "Holdings or the Borrower" in Section
4.02(A)(e) to read "the Borrower".

          2. Section 6 of the Credit Agreement is hereby amended by:

          (a) Changing each reference therein to "Holdings and the Borrower"
(other than in Section 6.09) or "Holdings or the Borrower" to read "the
Borrower";

          (b) Changing each other reference to "Holdings" in Section 6.01, 6.03,
6.04, 6.07, 6.08, (other than the reference therein to "Holdings," as to which
clause 2(d) applies) 6.12, 6.15, 6.16, 6.17 and 6.19 to read "the Borrower" or,
if any such reference is the first word in a sentence, to "The Borrower";

<PAGE>

          (c) Changing (x) the first reference to "Holdings" in Section 6.09 to
read "Holdings, the Borrower" and (y) the third reference to "Holdings" in such
Section to read "the Borrower" and adding after the second reference to
"Holdings" in such Section the phrase "(as to such projections and information
supplied by it)"; and

          (d) Deleting each reference to "Holdings," in Sections 6.12 and 6.13.

          3.    Section 7 of the Credit Agreement is hereby amended by:

          (a) Changing the initial reference therein to "Holdings and the
Borrower hereby covenant and agree" to read "The Borrower hereby covenants and
agrees";

          (b) Changing each reference therein to "Holdings or the Borrower" to
read "the Borrower";

          (c) Changing each other reference to "Holdings" in Sections 7.01,
7.02, 7.03, 7.04, 7.05, (other than the references therein to "Holdings," as to
which clause 3(d) applies) 7.07, 7.08, 7.09, (other than the first reference to
Holdings therein) 7.10(a) and 7.10(b) to read "the Borrower" or, if any such
reference is the first word in a sentence, to "The Borrower";

          (d) Deleting each reference to "Holdings," in Section 7.07; and

          (e) Changing the phrase "Holdings will cause the Borrower and the
Subsidiary Guarantors to grant" in Section 7.10(a) to read "The Borrower will,
and will cause the Subsidiary Guarantors to, grant".

          4.    Section 8 of the Credit Agreement is hereby amended by:

          (a) Changing the initial reference therein to "Holdings and the
Borrower hereby covenant and agree" to read "The Borrower hereby covenants and
agrees";

          (b) Changing each reference therein to "Holdings or the Borrower" to
read "the Borrower";

          (c) Deleting the reference to "(a)" in Section 8.01 and deleting all
of Section 8.01(b);

          (d) Changing each other reference to "Holdings" in Sections 8.02,
8.03, 8.04, 8.05, 8.06, 8.07, 8.08, 8.09(b), 8.10 and 8.14 to read "the
Borrower" or, if any such reference is the first word in a sentence, to "The
Borrower".

          (e) Changing Section 8.09(a) in its entirety to read:

          "(a) The Borrower will not, and will not permit any of its
Subsidiaries to, declare or pay any dividends (other than dividends payable
solely in capital stock of such Person) or return any capital to, its
stockholders, members and/or other owners or authorize or make any other
distribution, payment or delivery of property or cash to its stockholders,
members and/or other owners as such, or redeem, retire, purchase or otherwise
acquire, directly or indirectly, for

<PAGE>

a consideration, any shares of any class of its capital stock or other ownership
interests now or hereafter outstanding (or any warrants for or options or stock
appreciation rights in respect of any of such shares), or set aside any funds
for any of the foregoing purposes, or permit any of its Subsidiaries to purchase
or otherwise acquire for consideration any shares of any class of the capital
stock or other ownership interests of the Borrower or any other Subsidiary, as
the case may be, now or hereafter outstanding (or any options or warrants or
stock appreciation rights issued by such Person with respect to its capital
stock) (all of the foregoing "Dividends"), except that:

          (i)   any Subsidiary of the Borrower may pay Dividends or return
capital or make distributions and other similar payments with regard to its
capital stock or other membership interests to the Borrower or to a Subsidiary
of the Borrower;

          (ii)  the Borrower may redeem or repurchase its stock from officers,
employees and directors (or their estates) upon the death, permanent disability,
retirement or termination of employment of any such Person or otherwise in
accordance with any stock option plan or any employee stock ownership plan,
provided that (x) no Default or Event of Default is then in existence or would
--------
arise therefrom and (y) the aggregate amount of all cash paid in respect of all
such shares and interests so redeemed or repurchased in any calendar year does
not exceed $5,000,000;

          (iii) the Special Dividend shall be permitted; and

          (iv)  the Additional Dividend shall be permitted."; and

          (f)   Replacing the comma prior to clause (iv) in Section 8.14 with an
"and" and deleting all of Section 8.14 commencing with the phrase "and (v)".

          5.     Section 9 of the Credit Agreement is hereby amended by:

          (a)  Changing each reference to "Holdings" in Sections 9.04, 9.05 and
9.09 to read "the Borrower" or, if any such reference is the first word in a
sentence, to "The Borrower"; and

          (b)  Deleting all references to "Holdings," in Section 9.06; and

          (c)  Deleting the reference to "and Holdings" in the text of Section 9
immediately following Section 9.10.

          6.     Section 10 of the Credit Agreement is hereby amended by:

          (a)  Changing all references to "Holdings" in the definitions of
Acquisition Expenditures, Asset Sale, Deemed ECF, Excess Cash Flow, Material
Adverse Effect, (other than any reference therein to "Holdings," as to which
clause 6(f) applies) Multiemployer Plan, Plan, Reinvestment Prepayment Amount
and Subsidiary to read "the Borrower";

          (b)  Deleting the phrases "Holdings or" and "Holdings or by" in the
definition of Authorized Officer.

<PAGE>

          (c) Changing the definition of Change of Control in its entirety to
read:

          "Change of Control" shall mean, at any time and for any reason
whatsoever, (a) Carlyle and Carlyle Affiliates shall cease to have beneficial
ownership (within the meaning of Rule 13d-3 of the Exchange Act) on a fully
diluted basis in the aggregate of at least 30% of the voting interests in the
Borrower's capital stock or such higher percentage that exceeds the highest
percentage of the Borrower's capital stock owned by any other person or group
(as defined in Section 13(d) of the Exchange Act) or (b) the Board of Directors
of the Borrower shall cease to consist of a majority of Continuing Directors."

          (d) Changing the definition of Continuing Directors in its entirety to
read:

          "Continuing Directors" shall mean the directors of the Borrower on the
IPO Amendment Effective Date and each other director if such director's
nomination for the election to the Board of Directors of the Borrower is
recommended by a majority of the then Continuing Directors.

          (e) Changing the definition of EP Percentage in its entirety to read:

          "EP Percentage" shall mean in the case of (i) equity of the Borrower
issued to, or equity contributions made in the Borrower by, (A) Persons (or
controlling or controlled affiliates thereof) who are owners of the equity of
Holdings or the Borrower on the Initial Borrowing Date and/or are management
personnel of the Borrower and its Subsidiaries and/or (B) any Person (whether or
not an existing shareholder of the Borrower or Holdings) the proceeds of which
are to be utilized solely for investments in privatizations, co-productions,
Joint Ventures and Foreign Subsidiaries, to make Permitted Acquisitions and/or
to make Capital Expenditures, 0% and (ii) all other equity issuances and equity
contributions, 100%.

          (f) Deleting the phrase "Holdings," from the definitions of Credit
Party, ERISA Affiliate, Foreign Pension Plan and Multiemployer Plan;

          (g) Deleting the phrase "Holdings and" from the definition of
Guarantor;

          (h) Deleting the phrase "and include each of the Holdings Guaranty
and" from the definition of Guaranty;

          (i) Deleting the definition of Holdings Guaranty in its entirety; and

          (j) Adding the following new definitions in appropriate alphabetical
order:

          "IPO Amendment" shall mean the Amendment to this Agreement dated as of
October 15, 2001.

          "IPO Amendment Effective Date" shall mean the Effective Date as
defined in the IPO Amendment.

          7.   Section 11 of the Credit Agreement is hereby amended by changing
each reference therein to "Holdings" to read "the Borrower".

<PAGE>

          8.   Section 12 of the Credit Agreement is hereby amended by:

          (a) Changing each reference to "Holdings" in Sections 12.01 and
12.07(a) to read "the Borrower";

          (b) Changing the reference in Section 12.04(a) to "neither the
Borrower nor Holdings may assign" to read "the Borrower may not assign";

          (c) Changing the reference in the last sentence of Section 12.15 to
"The Borrower and Holdings hereby agree" to read "The Borrower hereby agrees".

          9.   Section 13 of the Credit Agreement is hereby deleted in its
entirety.

          10.  All of the Borrower's capital stock pledged by Holdings pursuant
to the Pledged Agreement is hereby released and the Collateral Agent is directed
to return same to Holdings as provided in the Pledge Agreement.

          11.  Holdings is hereby released from and shall cease to constitute a
party to each Credit Document to which it has been party.

          12.  Holdings hereby irrevocably and unconditionally waives its right
to consent to each future amendment to the Credit Agreement that is consented to
by the Borrower provided that any such amendment does not impose any
restrictions or obligations on Holdings.

          13.  In order to induce the Lenders to enter into this Amendment, the
Borrower hereby represents and warrants that:

          (a)  no Default or Event of Default exists as of the Effective Date
(as hereinafter defined), both before and after giving effect to this Amendment;
and

          (b)  all of the representations and warranties contained in the Credit
Agreement and the other Credit Documents are true and correct in all material
respects as of the Effective Date, with the same effect as though such
representations and warranties had been made on and as of the Effective Date (it
being understood that any representation or warranty made as of a specific date
shall be true and correct in all material respects as of such specific date).

          14.  This Amendment is limited as specified and shall not constitute a
modification, acceptance or waiver of any other provision of the Credit
Agreement or of any other Credit Document.

          15.  This Amendment may be executed in any number of counterparts and
by the different parties hereto on separate counterparts, each of which
counterparts when executed and delivered shall be an original, but all of which
shall together constitute one and the same instrument. A complete set of
counterparts shall be lodged with the Borrower and the Administrative Agent.

<PAGE>

          16.  THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE LAWS OF THE
STATE OF NEW YORK.

          17.  From and after the Effective Date, all references in the Credit
Agreement and each of the other Credit Documents to the Credit Agreement shall
be deemed to be references to the Credit Agreement as modified hereby.

          18.  Any provision of this Amendment which is prohibited or
unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective
to the extent of such prohibition or unenforceability without invalidating or
rendering unenforceable the remaining provisions hereof, and any such
prohibition or unenforceability in any jurisdiction shall not invalidate or
render unenforceable such provision in any other jurisdiction.

19.  This Amendment shall become effective at 00:01A.M. on the first date (the
"Effective Date") upon which the underwriters shall purchase the Borrower's
capital stock in connection with the IPO if each of Holdings, the Borrower and
the Required Lenders shall have theretofore signed a counterpart hereof (whether
the same or different counterparts) and shall have delivered (including by way
of facsimile transmission) the same to the Administrative Agent at the Notice
Office.

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