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

The significant event that occurred on September 11, 2001, in the United States was the terrorist attacks. On that day, a coordinated series of attacks were carried out by the extremist group al-Qaeda, which involved hijacking four commercial airliners. Two of the planes were flown into the World Trade Center towers in New York City, causing their eventual collapse, while a third plane struck the Pentagon in Arlington, Virginia. The fourth plane, United Airlines Flight 93, was headed for a target in Washington, D.C., but passengers attempted to regain control, leading to it crashing into a field in Pennsylvania.

This event had profound implications for national security policy and initiated a global response to terrorism, including the U.S. invasion of Afghanistan and the establishment of the Department of Homeland Security. The impact of the attacks was felt in numerous aspects of life in America and around the world, altering international relations and shaping public perception regarding security and terrorism.