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 constructed by contractor John McShain. Ground was broken on 11 September 1941, and the building was dedicated on 15 January 1943. General Brehon Somervell supplied the important 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 6,500,000 sq feet (600,000 m2) of space, of which 3,700,000 sq feet (340,000 m2) are used as offices. Some 23,000 military and civilian employees, and another 3,000 non-defense support employees, 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) of corridors. The fundamental 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.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. It was the first major foreign attack on Washington's governmental facilities since the town 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.