The Department is the largest employer in the world, with nearly 1.3 million active duty servicemen and women as of 2016. Adding to its employees are over 801,000 National Guardsmen and Reservists from the four services and it is headquartered at the Pentagon in Arlington, Virginia, just outside of Washington, D. C. The Department of Defense is headed by 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, the history of the defense of the United States started with the Continental Congress in 1775. The creation of the United States Army was enacted on 14 June 1775 and this coincides with the American holiday Flag Day. The Second Continental Congress would charter the United States Navy, on 13 October 1775, today, both the Navy and the Marine Corps are separate military services subordinate to the Department of the Navy. The Preamble of the United States Constitution gave the authority to federal government, to defend its citizens and this first Congress had a huge agenda, that of creating legislation to build a government for the ages. Legislation to create a military defense force stagnated, two separate times, President George Washington went to Congress to remind them of their duty to establish a military. In a special message to Congress on 19 December 1945, the President cited both wasteful military spending and inter-departmental conflicts, deliberations in Congress went on for months focusing heavily on the role of the military in society and the threat of granting too much military power to the executive. The act placed the National Military Establishment under the control of a single Secretary of Defense, the National Military Establishment formally began operations on 18 September, the day after the Senate confirmed James V. Forrestal as the first Secretary of Defense. The National Military Establishment was renamed the Department of Defense on 10 August 1949, under the Department of Defense Reorganization Act of 1958, channels of authority within the department were streamlined, while still maintaining the authority of the Military Departments. Also provided in this legislation was a centralized authority, the Advanced Research Projects Agency. The Act moved decision-making authority from the Military Departments to the Joint Chiefs of Staff and it also strengthened the command channel of the military over U. S. forces from the President to the Secretary of Defense. Written and promoted by the Eisenhower administration, it was signed into law 6 August 1958, because the Constitution vests all military authority in Congress and the President, the statutory authority of the Secretary of Defense is derived from their constitutional authorities. Department of Defense Directive 5100.01 describes the relationships within the Department. The latest version, signed by former Secretary of Defense Robert Gates in December 2010, is the first major re-write since 1987, the Office of the Secretary of Defense is the Secretary and Deputy Secretarys civilian staff. S. Government departments and agencies, foreign governments, and international organizations, OSD also performs oversight and management of the Defense Agencies and Department of Defense Field Activities. OSD also supervises the following Defense Agencies, Several defense agencies are members of the United States Intelligence Community and these are national-level intelligence services that operate under the jurisdiction of the Department of Defense but simultaneously fall under the authorities of the Director of National Intelligence
The Pentagon, headquarters of the U.S. Department of Defense
President Harry Truman signs the National Security Act Amendment of 1949
Department of Defense organizational chart (December 2013)
Joint Chiefs of Staff/Joint Staff organizational chart.