They are backed, in case of emergency, by the 1.2 million members of the Reserve and National Guard. In addition, there are about 670,000 civilian employees in the Defense Department. Under the President, who is also Commander in Chief, the Secretary of Defense exercises authority, direction, and control over the Department, which includes the separately organized military departments of Army, Navy, and Air Force, the Joint Chiefs of Staff providing military advice, the combatant commands, and defense agencies and field activities established for specific purposes.
The National Security Act Amendments of 1949 redesignated the National Military Establishment as the Department of Defense and established it as an executive department (10 U.S.C. 111), with the Secretary of Defense as its head.