MacArthur led the Allied forces in the Southwest Pacific during World War II, and after the war was in charge of the occupation of Japan. When North Korea invaded South Korea in June 1950, starting the Korean War, he was designated commander of the United Nations forces defending South Korea. He conceived and executed the amphibious assault at Inchon on 15 September 1950, for which he was hailed as a military genius. However, when he followed up his victory with a full-scale invasion of North Korea on Truman's orders, China intervened in the war and inflicted a series of defeats, compelling him to withdraw from North Korea. By April 1951, the military situation had stabilized, but MacArthur's public statements became increasingly irritating to Truman, and he relieved MacArthur of his commands. The Senate Armed Services Committee and the Senate Foreign Relations Committee held a joint inquiry into the military situation and the circumstances surrounding MacArthur's relief, and concluded that "the removal of General MacArthur was within the constitutional powers of the President but the circumstances were a shock to national pride."

An apolitical military was an American tradition, but one that was difficult to uphold in an era when American forces were employed overseas in large numbers. The principle of civilian control of the military was also ingrained, but the rising complexity of military technology led to the creation of a professional military. This made civilian control increasingly problematic when coupled with a cumbersome constitutional division of powers between the President as commander-in-chief, and the Congress with its power to raise armies, maintain a navy, and wage wars. In relieving MacArthur for failing to "respect the authority of the President" by privately communicating with Congress, Truman upheld the President's role as preeminent.