The United States Department of Defense (USDOD, DOD or DoD, initially briefly referred to as the National Military Establishment or NME) is the U.S. federal department allocated the largest level of budgetary resources and charged with coordinating and supervising all agencies and functions of the government relating directly to national security and the United States armed forces. The Department of Defense is an evolution of the United States Department of War. The organization and functions of the DoD are set forth in Title 10 of the United States Code of the United States Code.

The DoD is the major tenant of The Pentagon building near Washington, D.C., and has three main components—the United States Department of the Army, United States Department of the Navy, and United States 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 Jointness service schools, including the National War College.