What major event happened on September 11, 2001 in the United States?

What does this answer mean?

On September 11, 2001, terrorists attacked the United States. USCIS accepts four answers: terrorists attacked the United States; terrorists took over two planes and crashed them into the World Trade Center in New York City; terrorists took over a plane and crashed into the Pentagon in Arlington, Virginia; or terrorists took over a plane originally aimed at Washington, D.C., and crashed in a field in Pennsylvania.

That morning, 19 hijackers trained by the terrorist group al Qaeda took over four passenger planes. Two struck the twin towers of the World Trade Center, and a third struck the Pentagon, the headquarters of the U.S. military. On the fourth, United Flight 93, passengers fought back, and the plane crashed into a field in Somerset County, Pennsylvania, before it could reach its target in Washington.

Nearly 3,000 people were killed, more than died in the attack on Pearl Harbor in 1941.

This is one of the 20 questions on the 65/20 list. At the interview, the shortest answer is "terrorists attacked the United States."