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

On September 11, 2001, the United States experienced a series of coordinated terrorist attacks carried out by the extremist group al-Qaeda. This event involved hijacking four commercial airplanes, two of which were flown into the Twin Towers of the World Trade Center in New York City, resulting in their collapse. A third plane struck the Pentagon, the headquarters of the U.S. Department of Defense, while the fourth aircraft, United Flight 93, crashed in Pennsylvania after passengers attempted to regain control from the hijackers.

These attacks had profound implications for national security, foreign policy, and U.S. domestic life, leading to the initiation of the War on Terror, significant changes in security protocols, and an increased focus on combating terrorism both at home and abroad.

Other historical events listed, such as the Great Depression, Civil War, and the end of World War II, occurred in different time periods and do not relate to the significant and immediate impact felt on September 11, 2001.