Where is the headquarter of the US defence department?

Virginia

On 11 September 2001, nineteen hijackers hailing from a number of Arab countries took control of four American commercial aircraft shortly after takeoff and flew them into important buildings in the US. One airliner each crashed into the North and South Towers of the World Trade Centre in New York. A third aircraft crashed into the Pentagon building in Arlington, Virginia, where the US Defence Department is headquartered. The fourth aircraft, presumably bound for the Capitol building of the US Congress, came down in a field in Pennsylvania. The attacks have come to be known as “9/11”