<PAGE>

                                                                    EXHIBIT 4.15

                              DECLARATION OF TRUST


         DECLARATION OF TRUST, dated as of June 20, 2003, among American
Financial Group, Inc., an Ohio corporation, as Sponsor, The Bank of New York
(Delaware), a Delaware banking corporation, as Delaware Trustee, The Bank of New
York, a New York banking corporation, as Property Trustee, and James C. Kennedy
and Thomas E. Mischell as Regular Trustees (collectively with the Delaware
Trustee and the Property Trustee, the "Trustees"). The Sponsor and the Trustees
hereby agree as follows:

         1. The trust created hereby (the "Trust") shall be known as "American
Financial Capital Trust III", in which name the Trustees, or the Sponsor to the
extent provided herein, may conduct the business of the Trust, make and execute
contracts, and sue and be sued.

         2. The Sponsor hereby assigns, transfers, conveys and sets over to the
Trustees the sum of Ten Dollars ($10.00). The Trustees hereby acknowledge
receipt of such amount in trust from the Sponsor, which amount shall constitute
the initial trust estate. The Trustees hereby declare that they will hold the
trust estate in trust for the Sponsor. It is the intention of the parties hereto
that the Trust created hereby constitute a statutory trust under Chapter 38 of
Title 12 of the Delaware Code, 12 Del. C. Section 3801 et seq. (the "Statutory
Trust Act"), and that this document constitute the governing instrument of the
Trust. The Trustees are hereby authorized and directed to execute and file a
certificate of trust with the Delaware Secretary of State in the form attached
hereto.

         3. The Sponsor and the Trustees will enter into an amended and restated
Declaration of Trust, satisfactory to each such party and substantially in the
form to be included as an Exhibit to the Registration Statement referred to
below, to provide for the contemplated operation of the Trust created hereby and
the issuance of the Preferred Securities and Common Securities referred to
therein. Prior to the execution and delivery of such amended and restated
Declaration of Trust, the Trustees shall not have any duty or obligation
hereunder or with respect to the trust estate, except as otherwise required by
applicable law or as may be necessary to obtain prior to such execution and
delivery and licenses, consents or approvals required by applicable law or
otherwise.

         4. The Sponsor and the Regular Trustees hereby authorize and direct the
Sponsor, in each case on behalf of the Trust as the sponsor of the Trust, (i) to
execute and file with the Securities and Exchange Commission (the "Commission")
(a) a Registration Statement on Form S-3 (the "Registration Statement") and any
pre-effective or post-effective amendments to such Registration Statement,
relating to the registration under the Securities Act of 1933, as amended, of
the Preferred Securities of the Trust, and (b) a Registration Statement on Form
8-A (the "1934 Act Registration Statement") (including all pre-effective and
post-effective amendments thereto) relating

<PAGE>

to the registration of the Preferred Securities of the Trust under Section 12(b)
of the Securities Exchange Act of 1934, as amended; (ii) to execute and file
with the New York Stock Exchange, the American Stock Exchange or such other
national securities exchange or the Nasdaq National Market System as the Sponsor
shall determine, a listing or other similar application and all other
applications, statements, certificates, agreements and other instruments as
shall be necessary or desirable to cause the Preferred Securities to be listed
or approved for quotation on the New York Stock Exchange, the American Stock
Exchange or such other national securities exchange or the Nasdaq National
Market System; (iii) to execute and file such applications, reports, surety
bonds, irrevocable consents, appointments of attorneys for service of process
and other papers and documents as shall be necessary or desirable to register
the Preferred Securities under the securities or "Blue Sky" laws of such
jurisdictions as the Sponsor, on behalf of the Trust, may deem necessary or
desirable; and (iv) to negotiate and execute an underwriting agreement among the
Trust, the Sponsor, an investment banking firm or firms of national reputation
and the other parties thereto relating to the offer and sale of the Preferred
Securities. It is hereby acknowledged and agreed that in connection with any
execution, filing or document referred to in clauses (i)-(iii) above, (A) any
Regular Trustee (or his attorneys-in-fact and agents or the Sponsor as permitted
herein) is authorized on behalf of the Trust to file and execute such document
on behalf of the Trust, provided that the Registration Statement shall be signed
by all of the Regular Trustees, and (B) the Property Trustee and the Delaware
Trustee shall not be required to join in any such filing or execute on behalf of
the Trust any such document unless required by the rules and regulations of the
Commission, the New York Stock Exchange (or such other national securities
exchange or the Nasdaq National Market System) or state securities or "Blue Sky"
laws, and in such case only to the extent so required. In connection with all of
the foregoing, each Regular Trustee, solely in its capacity as Trustee of the
Trust, hereby constitutes and appoints Karl J. Grafe and Fred J. Runk, or either
of them, as his true and lawful attorney-in-fact and agent, with full power of
substitution and resubstitution, for such Regular Trustee or in the Regular
Trustee's name, place and stead, in any and all capacities, to sign the
Registration Statement, the 1934 Act Registration Statement and any and all
amendments (including post-effective amendments) thereto, and to file the same,
with all exhibits thereto, and other documents in connection therewith, with the
Commission, granting unto said attorney-in-fact and agent full power and
authority to do and perform each and every act and thing requisite and necessary
to be done in connection therewith, as fully to all intents and purposes as such
Regular Trustee might or could do in person, hereby ratifying and confirming all
that said attorney-in-fact and agent, or his substitute or substitutes, shall do
or cause to be done by virtue hereof.

         5. This Declaration of Trust may be executed in one or more
counterparts.

         6. The number of Trustees initially shall be four (4) and thereafter
the number of Trustees shall be such number as shall be fixed from time to time
by a written instrument signed by the Sponsor which may increase or decrease the
number of Trustees; provided that, to the extent required by the Statutory Trust
Act, one Trustee shall be an entity that has its principal place of business in
the State of Delaware. Subject to the foregoing, the Sponsor is entitled to
appoint or remove without cause any

<PAGE>

Trustee at any time. The Trustees may resign upon thirty days prior notice to
the Sponsor.

         IN WITNESS WHEREOF, the parties hereto have caused this Declaration of
Trust to be duly executed as of the day and year first above written.

                                     AMERICAN FINANCIAL GROUP, INC.,
                                     as Sponsor



                                     By:      /s/ James C. Kennedy
                                          -------------------------------------
                                     Name: James C. Kennedy
                                     Title: Vice President


                                     THE BANK OF NEW YORK (Delaware),
                                     as Delaware Trustee



                                     By:    /s/ William T. Lewis
                                          -------------------------------------
                                     Name:  William T. Lewis
                                     Title: Sr. Vice President


                                     THE BANK OF NEW YORK,
                                     as Property Trustee



                                     By:    /s/ Paul Schmalzel
                                          -------------------------------------
                                     Name:  Paul Schmalzel
                                     Title: Vice President



                                        /s/ James C. Kennedy
                                     ------------------------------------------
                                     James C. Kennedy, as Trustee



                                       /s/ Thomas E. Mischell
                                     ------------------------------------------
                                     Thomas E. Mischell, as Trustee