The September 11 attacks (also called 9/11) were four terrorist attacks against the United States of America. They all happened on the morning of Tuesday, September 11, 2001. The attacks killed almost 3,000 people, including the 19 attackers, making it the deadliest recent terrorist attack. They caused more than $10 billion in damage to infrastructure. They were carried out by the Islamic terrorist group al-Qaeda. The terrorists took control of 4 passenger airplanes to destroy 3 famous buildings, the Twin Towers and part of the Pentagon, by flying the planes into them. There were two attacks in New York City and one in Arlington, Virginia. The fourth attack, aimed at Washington, D.C. did not work and the plane crashed in a field near Shanksville, Pennsylvania.

The buildings attacked were the World Trade Center's twin towers in New York City, and the Pentagon in Arlington, Virginia. The fourth plane crashed in an empty field in Pennsylvania before it could reach its target in Washington, D.C. That target was either the White House or the United States Capitol. After the event, the United States government said the people who had done the attacks were close to the terrorist group al-Qaeda.

The September 11 attacks are commonly attributed as the event that made the War on Terrorism begin.