The Towers aflame during the attacks.
On September 11, 2001, the Islamist terrorist organization al-Qaeda, led by Osama bin Laden, executed a plan in which a group of nineteen men, mostly from Saudi Arabia, hijacked four passenger airplanes in order to crash two into New York City's World Trade Center twin towers, one into the Pentagon, and one into either the Capitol Building or the White House. The first three succeeded, but the fourth crashed in Shanksville, Pennsylvania.
The death toll has been established at 2,996 people, including the 19 hijackers, making it the worst terrorist attack in American history. While the international community condemned the attacks, some Islamic fundamentalists in the Middle East viewed them as part of a legitimate jihad. As a result, the United States invaded Afghanistan to eliminate al-Qaeda and bin Laden and to remove the Taliban regime (in retaliation for harboring bin Laden). The U.S. military subsequently occupied that country until December 2014, making it the longest war in America's history.
Besides unspeakable horror, this plan also unleashed a plethora of conspiracy theories, especially after President George W. Bush announced plans to intervene in Afghanistan.