The 9/11 tragedy refers to the terrorist attacks that took place in the United States on September 11, 2001. On that day, 19 terrorists associated with 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, causing both towers to collapse. A third plane was crashed into the Pentagon, the headquarters of the U.S. Department of Defense, near Washington, D.C. The fourth plane, United Airlines Flight 93, was headed toward another target, likely the U.S. Capitol or the White House, but passengers fought back against the hijackers, and it crashed in a field in Pennsylvania.

The attacks killed nearly 3,000 people and caused significant destruction, leading to a profound impact on the U.S. and the world. It resulted in increased global security measures, the initiation of the War on Terror, and the eventual wars in Afghanistan and Iraq. The event is commemorated each year, and the 9/11 Memorial in New York City stands as a tribute to those who lost their lives.