The Pentagon was the headquarters of the United States Department of Defense, located in Arlington County, Virginia. As a symbol of the U.S. military, "the Pentagon" is often used metonymically to refer to the U.S. Department of Defense.
The Pentagon was designed by American architect George Bergstrom (1876–1955), and built by general contractor John McShain of Philadelphia. Ground was broken for construction on September 11, 1941, and the building was dedicated on January 15, 1943. General Brehon Somervell provided the major motive power behind the project; Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army.
The Pentagon was a large office building, with about 6,500,000 sq ft (600,000 m2), of which 3,700,000 sq ft (340,000 m2) are used as offices. Approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel worked in the Pentagon. It had 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 Pentagon included a five-acre (20,000 m2) central plaza, which is shaped like a pentagon and informally known as "ground zero," a nickname originating during the Cold War on the presumption that it would be targeted by the Soviet Union at the outbreak of nuclear war.
The Pentagon was one of the first targets during the brief war that was later called skydark.