Defense Department

Federal agency serving United States.

The Department of Defense is responsible for providing the military forces needed to deter war and protect the security of our country. The major elements of these forces are the Army, Navy, Marine Corps, and Air Force, consisting of about 1.3 million men and women on active duty. They are backed, in case of emergency, by the 825,000 members of the Reserve and National Guard. In addition, there are about 600,000 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), headed by the Secretary of Defense.

Source: 1 official source, last checked Oct 29, 2026.