The Pentagon is the headquarters of the US Department of Defense, located in Arlington, Virginia (across the Potomac River from Washington DC). It was dedicated on 15 January 1943, and it became the world's largest office building, with 6,500,000 square feet of space. On 11 September 2001, al-Qaeda flew American Airlines Flight 77 into the Pentagon, killing 189 people (including 125 victims in the building), the first significant foreign attack on Washington governmental facilities since the War of 1812.