The Pentagon

With war approaching in 1941, the War Department's staff was scattered across more than a dozen buildings in Washington. Construction of a single headquarters across the Potomac began on September 11, 1941, and the Pentagon was completed in January 1943 after just sixteen months. Its five-sided shape came from the original site's boundaries and was kept when the location moved. With about 6.5 million square feet, it remains one of the largest office buildings in the world.

The building houses the Office of the Secretary of Defense, the Joint Staff, and the headquarters of the Army, Navy, Air Force, Marine Corps, and Space Force, with roughly twenty-five thousand military and civilian employees. Its five concentric rings and five floors are designed so that any two points can be reached on foot in a few minutes. The Pentagon Force Protection Agency guards it and the surrounding reservation.

On September 11, 2001, exactly sixty years after construction began, hijackers flew American Airlines Flight 77 into the building's west side, killing 184 people in the building and on the plane. The damaged section was rebuilt within a year, and a memorial opened on the site in 2008. The name Pentagon is widely used to mean the Defense Department and military leadership as a whole.