The September 11, 2001 attacks (often referred to as 9/11—pronounced "nine eleven") consisted of a series of coordinated terrorist suicide attacks by Islamic extremists on the United States of America on September 11, 2001.

It is believed that the attacks were originally planned to be executed between March and September 2000 but eventually occurred on the morning of September 11, 2001, when nineteen terrorists affiliated with al-Qaeda hijacked four commercial passenger jet airliners. Each team of hijackers included a trained pilot. The hijackers intentionally crashed two of the airliners (United Airlines Flight 175 and American Airlines Flight 11) into the World Trade Center in New York City, one plane into each tower (1 WTC and 2 WTC), resulting in the collapse of both buildings soon afterward and irreparable damage to nearby buildings. The hijackers crashed a third airliner (American Airlines Flight 77) into the Pentagon in Arlington County, Virginia, near Washington, D.C. Passengers and members of the flight crew on the fourth aircraft (United Airlines Flight 93) attempted to retake control of their plane from the hijackers; that plane crashed into a field near the town of Shanksville in rural Somerset County, Pennsylvania. In addition to the 19 hijackers, 2,974 people died; another 24 are missing and presumed dead. The victims were predominantly civilians.