The United States was attacked on September 11 , 2001 by al Qaeda terrorists who hijacked commercial airliners and crashed them in coordinated strikes into both towers of the World Trade Center in Lower Manhattan , New York City and the Pentagon in Washington , D.C . The fourth plane crashed into a field in Shanksville , Pennsylvania , and its target was likely a prominent building in Washington , D.C. , most probably either the Capitol Building or the White House . Within three hours of the start of the first hijacking and two hours after American Airlines Flight 11 struck the World Trade Center , Rumsfeld raised the defense condition signaling of the United States offensive readiness to DEFCON 3 , the highest it had been since the Arab-Israeli war in 1973
American Airlines Flight 11
American Airlines Flight 11
Donald Rumsfeld