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

The significant event that occurred on September 11, 2001, was a series of coordinated terrorist attacks carried out by the extremist group al-Qaeda. On that day, four commercial airplanes were hijacked; two of them were flown into the Twin Towers of the World Trade Center in New York City, causing their eventual collapse, while a third was crashed into the Pentagon in Arlington, Virginia. The fourth plane, United Airlines Flight 93, was headed toward a target in Washington, D.C., but passengers attempted to retake control, leading to its crash in a field in Pennsylvania.

This date marked a pivotal moment in U.S. history, leading to significant changes in national security policies and initiating the War on Terror. The attacks resulted in approximately 3,000 deaths and left a lasting impact on American society, politics, and international relations. The choice highlighting these terrorist attacks accurately represents the nature and gravity of the event, as it became a catalyst for both national and global changes in the approach toward terrorism.