[4] He graduated from South Dakota State University in 1941 with a Bachelor of Science in economics,[2] and received a Reserve Officers' Training Corps commission as a second lieutenant of Infantry.

[4] Shortly after the United States' entry into World War II, DePuy was assigned in 1942 as a lieutenant, at age 22, to the newly formed 90th Infantry Division.

Van Fleet called DePuy's staff work as "brilliant" and added that his "inspiring and courageous leadership" of his battalion provided "some of the finest examples of infantry operations in this War".

[2] During the Korean War, DePuy spent time convalescing after a broken leg, and then performed clandestine service for the Central Intelligence Agency in China and other Asian countries.

DePuy met Marjory Kennedy Walker of Salem, Virginia, a Far East specialist who served with both the Office of Strategic Services and the Central Intelligence Agency,[8] and they were married in June 1951.

This led Army chief of staff General Harold K. Johnson to say, "If every division commander relieved people like DePuy, I'd soon be out of lieutenant colonels and majors.

[11] In another speech that year, he emphasized the need for realistic training in highly maneuverable combined arms formations with tanks playing a leading role on the battlefield.

"[6] This period of study resulted in TRADOC's first publication, a 1976 revision of the Army's FM 100-5 Operations, which promoted an attrition-based doctrine called "Active Defense".

[12] Its effect was:[6] In theory Active Defense was supposed to compensate for:[6] The 1976 version of Operations was the first in the series to incorporate force ratios as a decision-making tool.

The manual heavily emphasized the favorability of defending with a ratio of 3:1, mentioning it in five varying forms of application at both the tactical and the operational levels of war, similar to the argument advanced in Lanchester's Laws.