On Tuesday, September 11, 2001, al-Qaeda (an Islamist terrorist organization led by Osama bin Laden) executed a plan in which a group of (in the end) 19 men, mostly from Saudi Arabia, hijacked four passenger airplanes in order to crash two into the NYC World Trade Center's twin towers, one into the Pentagon, and one into either the Capitol Building or the White House. The first three succeeded; the fourth crashed near Shanksville, Pennsylvania.
The death toll has been established at 2,996 people, including the 19 hijackers, making it the worst terrorist attack in United States 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 of 9/11, the United States invaded Afghanistan to destroy Al-Qaeda and hunt down its leader Osama bin Laden. Its military subsequently occupied that country until December 2014, making it the longest war in U.S. history.
Besides unspeakable horror, this plan also unleashed a plethora of 9/11 conspiracy theories, especially after Bush announced plans to intervene in Afghanistan (at the time, harboring Al Qaeda's leadership) and later Iraq (which had no involvement at all in 9/11). 9/11 conspiracy theorists, known collectively as "truthers" make varying assertions, including: claiming that the attacks were condoned by the U.S. government, or even carried out by the government as a false flag operation, as a pretext for launching the War on Terror; claiming that Israel did the attacks to increase American support; claiming that the attacks were masterminded by an international Jewish conspiracy; claiming that they were carried out as part of an ongoing strategy to bring about the New World Order or the Illuminati.