Which 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 series of terrorist attacks. On that day, members of the extremist group Al-Qaeda hijacked four commercial airplanes. Two of these planes were flown into the Twin Towers of the World Trade Center in New York City, leading to their eventual collapse, while a third plane struck the Pentagon in Arlington, Virginia. The fourth plane, United Airlines Flight 93, was headed for another target in Washington, D.C., but crashed in Pennsylvania after passengers attempted to overcome the hijackers.

This tragic event resulted in the loss of nearly 3,000 lives and significant changes in U.S. national security policies, international relations, and the initiation of the War on Terror. The attacks are remembered as one of the most defining moments in recent U.S. history, marking a profound shift in domestic and foreign policy.

The other events listed, such as an economic recession, a presidential election, or a military conflict, did not occur on that specific date and do not represent the gravity and impact of the September 11 attacks on the nation and the world.