The United States Department of Defense (DOD or DoD) is the federal department charged with coordinating and supervising all agencies and functions of the government relating directly to national security and the military. The organization and functions of the DOD are set forth in Title 10 of the United States Code.

The DOD is the major tenant of The Pentagon building near Washington, D.C., and has three major components – the Department of the Army, the Department of the Navy, and the Department of the Air Force. Among the many DOD agencies are the Missile Defense Agency, the Defense Advanced Research Projects Agency (DARPA), the Pentagon Force Protection Agency (PFPA), the Defense Intelligence Agency (DIA), the National Geospatial-Intelligence Agency (NGA), and the National Security Agency (NSA). The department also operates several joint service schools, including the National War College.

During 1945, specific plans for the proposed DoD were put forth by the Army, the Navy, and the Joint Chiefs of Staff. In a special message to Congress on 19 December 1945, President Harry Truman proposed creation of a unified Department of National Defense. A proposal went to Congress in April 1946, but was held up by the Naval Affairs Committee hearings in July 1946, which raised objections to the concentration of power in a single department. Truman eventually sent new legislation to Congress in February 1947, where it was debated and amended for several months.