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 or synecdoche 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; 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 6.5 million square feet of floor space, of which 3.7 million sq ft are used as offices. Some 23,000 military and civilian employees, 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 miles of corridors. The central five-acre pentagonal plaza is nicknamed "ground zero" on the presumption that it would be a prime target in a nuclear war.
On 11 September 2001, American Airlines Flight 77 was hijacked and flown into the western side of the building, killing 189 people. Of those killed, 64 were on the hijacked airplane, and 125 were in the Pentagon. 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.