Which of the following presidents did not serve in the U.S. Air Force?

The correct response is tied to the military service history of Harry S. Truman. He did not serve in the U.S. Air Force because it was not established until after World War II, specifically in 1947. Truman served during World War I, where he was a captain in the Field Artillery.

In contrast, the other presidents mentioned were involved with the military branches that either preceded the Air Force or aligned with its formation. George H.W. Bush served as a naval aviator in the U.S. Navy during World War II. Dwight D. Eisenhower was a five-star general in the U.S. Army and played a pivotal role in World War II before becoming president. Richard Nixon served in the Navy during World War II as well.

Understanding the timeline of military establishments and individual service histories helps clarify why Truman is the right choice in this context.