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

The significant event that occurred on September 11, 2001, was the terrorist attacks coordinated by the extremist group al-Qaeda. On that day, four commercial airplanes were hijacked; two were flown into the Twin Towers of the World Trade Center in New York City, causing the towers to collapse, and another was directed towards the Pentagon. The fourth plane, United Airlines Flight 93, was headed for a target in Washington, D.C., but passengers attempted to retake control, leading to it crashing in a field in Pennsylvania.

These attacks resulted in a profound loss of life, with nearly 3,000 people killed, and had long-lasting impacts on U.S. policy, national security, and global politics. The event is commemorated annually and is considered a pivotal moment in American history, marking the beginning of the War on Terror.

Other options, such as a stock market crash, the assassination of a president, or the invasion of a foreign country, do not relate to that date and do not capture the global ramifications and historical significance that the September 11 attacks initiated.