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

The significant event that occurred on September 11, 2001, was the terrorist attacks on the World Trade Center and the Pentagon. On that day, coordinated attacks by the extremist group al-Qaeda resulted in the hijacking of four commercial airplanes. Two of these planes were flown into the Twin Towers of the World Trade Center in New York City, causing their eventual collapse, while a third plane struck the Pentagon, the headquarters of the U.S. Department of Defense, located in Arlington, Virginia.

This event had profound implications for U.S. domestic and foreign policy, leading to the War on Terror, significant changes in national security measures, and the implementation of various legislation aimed at preventing future attacks. The repercussions of 9/11 are still felt today and have shaped the global geopolitical landscape. The other options listed do not pertain to this date or have an impact comparable to that of the September 11 attacks.