The September 11, 2001 attacks were a series of coordinated terrorist strikes carried out against the United States by the Islamist militant organization al-Qaeda. Nineteen hijackers seized four commercial aircraft; two were flown into the Twin Towers of the World Trade Center in New York City, one into the Pentagon in Arlington, Virginia, and a fourth crashed into a field near Shanksville, Pennsylvania after passengers attempted to overpower the hijackers. Approximately 2,977 people were killed, making it the deadliest terrorist attack carried out on U.S. soil and the deadliest foreign attack on American soil since Pearl Harbor.
Al-Qaeda, led by Osama bin Laden, had declared war on the United States in 1996 and 1998, citing American military presence in Saudi Arabia, U.S. policy toward Israel, and sanctions against Iraq as grievances. The 9/11 attacks were planned over several years, with operatives entering the United States to train as pilots at American flight schools. The operation was directed from Afghanistan, where al-Qaeda operated under the protection of the Taliban government.
On the morning of September 11, hijackers took control of American Airlines Flight 11, United Airlines Flight 175, American Airlines Flight 77, and United Airlines Flight 93. The North Tower (WTC 1) was struck at 8:46 a.m.; the South Tower (WTC 2) was struck at 9:03 a.m. The South Tower collapsed at 9:59 a.m. and the North Tower at 10:28 a.m. The Pentagon was struck at 9:37 a.m. Flight 93 crashed at 10:03 a.m. All 19 hijackers died, along with all passengers and crew aboard the four aircraft. The total death toll includes citizens of more than 90 countries.
U.S. airspace was cleared and commercial aviation grounded for the first time in history. President George W. Bush declared a national emergency. Congress passed the Authorization for Use of Military Force (AUMF) on September 18, 2001, enabling military action against those responsible. The USA PATRIOT Act was signed into law on October 26, 2001, expanding domestic surveillance authority. The Department of Homeland Security was created in 2002, restructuring federal emergency and security agencies.
The United States, with NATO allies, invaded Afghanistan in October 2001, toppling the Taliban government and pursuing al-Qaeda. The war in Afghanistan continued until the U.S. withdrawal and Taliban return to power in August 2021. The attacks also shaped the justification for the 2003 invasion of Iraq — see September 11th Attacks - Iraq War Connection Viewpoint.
The attacks prompted a broad expansion of domestic surveillance, counterterrorism law, and airport security. Debates over the proper scope of these measures continue — see September 11th Attacks - Civil Liberties Viewpoint. The treatment of detainees at Guantanamo Bay and the use of enhanced interrogation techniques remain subjects of ongoing controversy — see September 11th Attacks - Enhanced Interrogation Viewpoint.
First responders and recovery workers at Ground Zero experienced elevated rates of respiratory illness, cancer, and other long-term health conditions attributed to exposure at the site. Congress passed the James Zadroga 9/11 Health and Compensation Act in 2010, establishing a health program and victim compensation fund. For a fuller account of the long-term human consequences, see September 11th Attacks - Aftermath and Victims.
The 9/11 Commission, formally the National Commission on Terrorist Attacks Upon the United States, published its report in July 2004. Among its central findings was the failure of the CIA and FBI to share information about known al-Qaeda operatives prior to the attacks, a breakdown that allowed several hijackers to remain in the country undetected. The Commission made recommendations that shaped subsequent reorganization of U.S. intelligence. A portion of the report, the “28 Pages” concerning Saudi government connections, was classified until 2016. Questions about Saudi government involvement remain a matter of legal and political dispute — see September 11th Attacks - Saudi Involvement Viewpoint.
A range of alternative explanations for the attacks — including theories that the U.S. government had foreknowledge or direct involvement — have been advanced since 2001. These are examined separately — see September 11th Attacks - Conspiracy Theories Viewpoint.
For a detailed chronological account of the planning, execution, investigations, and long-term aftermath of the attacks, see September 11th Attacks - History.