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

The major event that occurred in the United States on September 11, 2001, was a series of coordinated terrorist attacks. On that day, members of the extremist group al-Qaeda hijacked four commercial airplanes and carried out suicide missions. Two of these planes were flown into the World Trade Center in New York City, leading to the collapse of both towers. A third plane struck the Pentagon in Arlington, Virginia, and the fourth plane, United Airlines Flight 93, crashed in Pennsylvania after passengers attempted to regain control from the hijackers.

This tragic event resulted in significant loss of life and profound impacts on national security policies, international relations, and the lives of millions of Americans. The attacks prompted the United States to initiate the War on Terror, leading to military engagements in Afghanistan and changes in domestic security measures.

The other options reflect events that are not associated with that specific date. A declaration of war or a presidential inauguration did not happen on September 11, 2001, and while natural disasters occur in the U.S., this particular date is most notable for the terrorist attacks that profoundly affected the nation and the world.