The Department of Defense is an executive branch department of the federal 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 servicemen and women as well as their support staff of civilian workers. Adding to the total is over 1.1 million National Guardsmen and Reservists from the four services, bringing the total to just over 3.2 million employees.
The Department of Defense is headed by the Secretary of Defense, a cabinet-level head who reports directly to the President of the United States. Beneath the Department of Defense, are 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. In addition, four national intelligence services are subordinate to the Department of Defense: 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 including the 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. Military operations are managed by nine regional or functional Unified Combatant Commands. The Department of Defense also operates several joint services schools, including the National Defense University (NDU) and the National War College (NWC).