On September 11, 2001, four commercial airplanes were hijacked by al-Qaeda terrorists. Two planes crashed into the World Trade Center towers in New York City, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed into a field in Pennsylvania. Nearly 3,000 people were killed during the 9/11 terrorist attacks, which triggered major U.S. initiatives to combat terrorism and defined the presidency of George W. Bush.