During the period 1940 to 1945, Clark served two tours of active duty in the Army with preceding and intervening private consulting work related to military supply and logistics.

Clark was involved in supply and ordnance matters during the North African campaign and served as Dwight D. Eisenhower's Deputy G-4 (assistant chief of staff for logistics) at SHAEF (Supreme Headquarters Allied Expeditionary Force).

In the late 1940s, in conjunction with a small group of liberal Republicans, including Russell Davenport and Senator James Duff, Clark worked for Eisenhower's presidential candidacy.

Discussions resulted in a plan to reorganize the Institute under the auspices of Columbia University and broaden its scope to include the study of war and peace.

Last, Clark served Eisenhower in the latter stages of the 1952 campaign by submitting a memorandum on U.S. Pacific policy, which he later elaborated to include mutual security matters.

The main facet of the project was the writing of his memoirs, but in preparing for the task he submerged himself in histories of the period, collected and organized primary documentation, and contacted people he had worked with in the course of his career.