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

Why this is the answer

On the morning of September 11, 2001, terrorists attacked the United States. They belonged to al Qaeda, a network of Islamic extremists, and they took over four passenger planes flying out of U.S. airports. Two were flown into the Twin Towers of the World Trade Center in New York City, and both buildings were destroyed. A third crashed into the Pentagon in Arlington, Virginia, near Washington, D.C. The fourth was originally aimed at Washington, but its passengers fought the hijackers, and the plane went down in a field near Shanksville, Pennsylvania. Nine days later, speaking to Congress, the President praised the courage of passengers who rushed the terrorists to save others on the ground. Almost 3,000 people died, most of them civilians. The USCIS study guide compares it to Pearl Harbor and calls it the worst attack on the American homeland since 1941. The official list gives four answers, and they work like a zoom lens. The first is the whole picture: terrorists attacked the United States. The other three each describe one of the three crash sites: New York, Arlington and Pennsylvania. Any one is correct, so the short version is the simplest to say with confidence. If you prefer a longer answer, get the place names right. The Pentagon is in Arlington, Virginia, not in Washington itself, and the plane that crashed in Pennsylvania never reached its target. The country's response began within days. Congress passed a joint resolution on September 14, and on September 18 it became law. It authorized force against the nations, organizations or persons who planned, authorized, committed or aided the attacks. That law led to the conflicts in the next question on the list, the War on Terror and the wars in Afghanistan and Iraq.