What major event occurred on September 11, 2001 in the United States?

The major event that occurred on September 11, 2001, in the United States was a terrorist attack. This tragic incident involved four commercial airplanes being hijacked by members of the extremist group al-Qaeda. Two of these planes were flown into the Twin Towers of the World Trade Center in New York City, leading to their collapse, while another plane hit the Pentagon, the headquarters of the U.S. Department of Defense. The fourth plane, United Airlines Flight 93, was headed toward a target in Washington, D.C., but passengers attempted to overcome the hijackers, resulting in the plane crashing in a field in Pennsylvania.

The significance of this event is profound, as it led to vast changes in U.S. domestic and foreign policy, including the implementation of the USA PATRIOT Act, the initiation of the War on Terror, and significant shifts in national security measures. It is remembered each year and has had a lasting impact on American society and global relations. This specific context and the depth of impact it had justify why the terrorist attack is recognized as the correct answer.