The Department of Defense (Defense Department, USDOD, DOD, DoD or the Pentagon) is the executive department of the government of the United States charged with coordinating and supervising all agencies and functions of the government concerned directly with national security and the United States Armed Forces. The Department is also the largest employer in the world,with more than 2.13 million active duty soldiers, sailors, marines, airmen, and civilian workers, and over 1.1 million National Guardsmen and members of the Army, Navy, Air Force, and Marine Reserves. The grand total is just over 3.2 million servicemen, servicewomen, and civilians.
The Department – headed by the Secretary of Defense – has three subordinate military departments: the U.S. Department of the Army, the U.S. Department of the Navy, and the U.S. Department of the Air Force which oversee the U.S. Army, U.S. Navy, U.S. Marine Corps, and the U.S. Air Force. In addition, four national intelligence services are subordinate to DOD – the Defense Intelligence Agency (DIA), the National Security Agency (NSA), the National Geospatial-Intelligence Agency (NGA), and the National Reconnaissance Office (NRO). Other Defense Agencies include Defense Advanced Research Projects Agency (DARPA), the Defense Logistics Agency (DLA), the Missile Defense Agency (MDA), Defense Threat Reduction Agency (DTRA), and the Pentagon Force Protection Agency (PFPA), all of which are under the command of the Secretary of Defense. DOD’s military operations are managed by nine regional or functional Unified Combatant Commands. DOD also operates several joint services schools, including the National Defense University (NDU) and the National War College (NWC).