one of the Cabinet departments, created in 1947. The department was formerly known as the Department of War, but later was combined with the Department of the Navy to create the Department of Defense by President Truman. The DOD provides military forces to protect the security of the United States and prevent war. It is headed by the Secretary of Defense, and includes: the Office of the Secretary of Defense; the Joint Chiefs of Staff; the unified combatant commands; the Army, Navy (including the Marines) and Air Force; and other offices and agencies.