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.