United States. Department of Defense

The Pentagon is the headquarters building of the United States Department of Defense. As a symbol of the U.S. military, the phrase The Pentagon is also often used as a metonym for the Department of Defense and its leadership.

Located in Arlington County, Virginia, across the Potomac River from Washington, D.C., the building was designed by American architect George Bergstrom and built by contractor John McShain. Ground was broken on 11 September 1941, and the building was dedicated on 15 January 1943. General Brehon Somervell provided the major motivating power behind the project;[5] Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army.

The Pentagon is the world's largest office building, with about 6,500,000 sq ft (600,000 m2) of space, of which 3,700,000 sq ft (340,000 m2) are used as offices.[6][7] Some 23,000 military and civilian employees,[7] and another 3,000 non-defense support personnel, work in the Pentagon. It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km)[7] of corridors. The central five-acre (20,000 m2) pentagonal plaza is nicknamed "ground zero" on the presumption that it would be a prime target in a nuclear war.[8]

On 11 September 2001, exactly 60 years after the building's construction began, American Airlines Flight 77 was hijacked and flown into the western side of the building, killing 189 people (59 victims and the five terrorists on board the airliner, as well as 125 victims in the building), according to the 9/11 Commission Report.[9] It was the first significant foreign attack on Washington's governmental facilities since the city was burned by the British during the War of 1812.

The Pentagon is listed on the National Register of Historic Places and is a National Historic Landmark.[3]

Proposals to coordinate the activities of the military services were initially considered by U.S. Congress in 1944. Specific plans were put forth in 1945 by the Army, the Navy, and the Joint Chiefs of Staff . In a special message to U.S. Congress on December 19, 1945, President Harry Truman proposed creation of a unified Department of National Defense. A proposal reached Congress in April 1946, but was held up by the Naval Affairs Committee held hearings in July 1946 due to objections to the concentration of power in a single department. Truman eventually sent new legislation to Congress in February 1947, where it was debated and amended for several months.

On July 26, 1947, Truman signed the National Security Act of 1947, which set up the National Military Establishment to begin operations on September 18, 1947, the day after the confirmation of James V. Forrestal as the first Secretary of Defense. The Establishment had the unfortunate abbreviation 'NME' (the obvious pronunciation being "enemy"), and was renamed the "Department of Defense" on August 10, 1949; in addition, the secretary was given greater authority over the military departments of the Army, Navy, and Air Force.

The Department was created by combining the War Department (founded in 1789) with the Navy Department (founded in 1798; formerly the Board of Admiralty, founded in 1780), and the newly created Department of the Air Force. The Department was formed in order to reduce interservice rivalry which was believed to have reduced military effectiveness during World War.

The command structure of the Department of Defense underwent several revisions but most recently was defined by the Goldwater-Nichols Act of 1986. Under the act, the chain of command ran from the President of the United States , through the Secretary of Defense, to the unified combatant commanders. The Chairman of the Joint Chiefs of Staff and the several Chiefs of Staff were responsible for readiness of the U.S. military and served as the President's military advisers, but were not in the chain of command. The Chairman of the Joint Chiefs of Staff was by law the highest ranking military officer in the United States.