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

On September 11, 2001, terrorists carried out a coordinated attack on the United States, which involved hijacking four commercial airplanes. Two of the planes were flown into the Twin Towers of the World Trade Center in New York City, causing both towers to collapse. A third plane was directed into the Pentagon, the headquarters of the U.S. Department of Defense. The fourth plane, United Airlines Flight 93, was also hijacked but passengers attempted to retake control from the hijackers, leading to its crash in a Pennsylvania field instead of a targeted location, likely the U.S. Capitol or the White House.

This event had profound effects on national security, foreign policy, and emergency response protocols in the U.S., and it ultimately led to the War on Terror and significant changes in legislation and military engagement around the world. The attacks are often referred to simply as "9/11" and are a pivotal moment in contemporary U.S. history, marking a shift in the nation's approach to terrorism and global conflicts.