He was appointed to be Secretary of Defense in September 1950

Another interesting fact is that he was appointed to be Secretary of Defense in September 1950. When the early months of the Korean War revealed the Defense Department's lack of preparedness, President Truman removed Secretary Louis A. Johnson and appointed Marshall as Secretary of Defense in September 1950. Because the National Security Act of 1947 forbade a uniformed military officer from serving in the post, the appointment required a congressional waiver. Marshall was included in this limitation because individuals promoted to General of the Army are not actually retired, but remain formally on active duty long after their active service has ended. Marshall was the first to be granted such a waiver; in 2017, Jim Mattis became the second, and General Lloyd Austin got the third in January 2021. Marshall's primary responsibility as Secretary of Defense was to restore trust and morale to the Defense Department while rebuilding the armed forces following their demobilization during World War II.

Marshall worked to increase manpower to meet the demands of both the Korean War and the European Cold War. Marshall assembled a new leadership team to carry out his aims, including Robert A. Lovett as his deputy and Anna M. Rosenberg, former chairman of the War Manpower Commission, as assistant secretary of defense for manpower. He also tried to repair the ties between the Defense and State Departments, as well as the Secretary of Defense and the Joint Chiefs of Staff.