Final answer:
Terrorists attacked New York and Washington, D.C on September 11, 2001, in the United States.
Step-by-step explanation:
The major event that happened on September 11, 2001, in the United States was that terrorists attacked New York and Washington, D.C. This event is commonly referred to as the 9/11 attacks. The attacks were carried out by the Islamic extremist group Al-Qaeda and resulted in the destruction of the World Trade Center towers in New York City and significant damage to the Pentagon in Washington, D.C.
Learn more about 9/11 attacks