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

On September 11, 2001, terrorists launched a coordinated series of attacks against the United States, which involved hijacking commercial airliners and crashing them into significant landmarks, including the World Trade Center in New York City and the Pentagon in Arlington, Virginia. This tragic event resulted in the deaths of nearly 3,000 people and prompted extensive security measures and changes in U.S. foreign policies, including the initiation of the War on Terror. The attacks on that day are often referred to as 9/11 and are marked as a pivotal moment in American history, affecting both the national psyche and global geopolitics.

The other options describe events that did not occur on that specific date. There was no major earthquake on the West Coast on September 11, 2001, nor did the day signify the beginning of an economic crisis or the formal declaration of war, although the aftermath of the attacks did lead to military actions in subsequent years.