What is 9/11 and why did it happen?

19 militants associated with the Islamic extremist group al Qaeda hijacked four airplanes and carried out attacks against targets in the United States. Two of the planes were flown into the twin towers of the World Trade Center in New York City, a third plane hit the Pentagon just outside Washington, D. C

9/11 was a terror attack on the twin towers in new York and also on the Pentagon

Explanation:

the terrorists group ISIS preformed the attack and killed thousands