The Pentagon was crashed into as part of 911.
September 11 attacks . were a series of four coordinated terrorist attacks by the Islamic terrorist group al-Qaeda on the United States on the morning of Tuesday , September 11 , 2001 . A third plane , American Airlines Flight 77 , was crashed into the Pentagon ( the headquarters of the United States Department of Defense ) in Arlington County , Virginia , leading to a partial collapse of the building 's western side .