On September 11, 2001, 19 militants associated with al-Qaeda hijacked four commercial aircraft in the United States. Two planes were flown into the twin towers of the World Trade Center in New York City, causing their collapse. A third plane hit the Pentagon, and a fourth crashed in a field in Pennsylvania after passengers attempted to overcome the hijackers. Nearly 3,000 people were killed. The attacks led to the War on Terror, the invasion of Afghanistan and Iraq, and sweeping changes to global security policies.
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