September 11 attacks

The September 11 attacks, also known as 9/11, were four coordinated Islamist terrorist suicide attacks by al-Qaeda against the United States in 2001. Nineteen terrorists hijacked four commercial airliners, crashing the first two into the Twin Towers of the World Trade Center in New York City and the third into the Pentagon (headquarters of the US Department of Defense) in Arlington County, Virginia. The fourth plane crashed in a rural Pennsylvania field during a passenger revolt. The September 11 attacks killed 2,977 people, making it the deadliest terrorist attack in history. In response to the attacks, the United States waged the multi-decade global war on terror to eliminate hostile groups deemed terrorist organizations, as well as the foreign governments purported to support them.

Ringleader Mohamed Atta flew American Airlines Flight 11 into the North Tower of the World Trade Center complex at 8:46 a.m.Around seventeen minutes later at 9:03 a.m.,[i] United Airlines Flight 175 hit the South Tower. Both collapsed within an hour and forty-two minutes,[j] destroying the remaining five structures in the complex. American Airlines Flight 77 crashed into the Pentagon at 9:37 a.m., causing a partial collapse. The fourth and final flight, United Airlines Flight 93, was believed by investigators to target either the United States Capitol or the White House. Alerted to the previous attacks, the passengers revolted against the hijackers who crashed the aircraft into a field near Shanksville, Pennsylvania, at 10:03 a.m. The Federal Aviation Administration ordered an indefinite ground stop for all air traffic in U.S. airspace, preventing any further aircraft departures until September 13 and requiring all airborne aircraft to return to their point of origin or divert to Canada. The actions undertaken in Canada to support incoming aircraft and their occupants were collectively titled Operation Yellow Ribbon.

That evening, the Central Intelligence Agency informed President George W. Bush that its Counterterrorism Center had identified the attacks as having been the work of al-Qaeda under Osama bin Laden. The United States responded by launching the war on terror and invading Afghanistan to depose the Taliban, which rejected U.S. terms to expel al-Qaeda from Afghanistan and extradite its leaders. NATO's invocation of Article 5 of the North Atlantic Treaty—its only usage to date—called upon allies to fight al-Qaeda. As U.S. and allied invasion forces swept through Afghanistan, bin Laden eluded them. He denied any involvement until 2004, when excerpts of a taped statement in which he accepted responsibility for the attacks were released. Al-Qaeda's cited motivations included U.S. support of Israel, the presence of U.S. military bases in Saudi Arabia and sanctions against Iraq. The nearly decade-long manhunt for bin Laden concluded in May 2011, when he was killed during a U.S. military raid on his compound in Abbottabad, Pakistan. The War in Afghanistan continued for another eight years until the agreement was made in February 2020 for American and NATO troops to withdraw from the country.

The attacks killed 2,977 people, injured thousands more[k] and gave rise to substantial long-term health consequences while also causing at least $10 billion in infrastructure and property damage. It remains the deadliest terrorist attack in history as well as the deadliest incident for firefighters and law enforcement personnel in American history, killing 343 and 72 members, respectively. The loss of life stemming from the impact of Flight 11 made it the most lethal multi-plane crash in aviation history followed by the death toll incurred by Flight 175. The destruction of the World Trade Center and its environs seriously harmed the U.S. economy and induced global market shocks. Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies. The total number of deaths caused by the attacks, combined with the death tolls from the conflicts they directly incited, has been estimated by the Costs of War Project to be over 4.5 million. 

Cleanup of the World Trade Center site (colloquially "Ground Zero") was completed in May 2002, while the Pentagon was repaired within a year. After delays in the design of a replacement complex, six new buildings were planned to replace the lost towers, along with a museum and memorial dedicated to those who were killed or injured in the attacks. The tallest building, One World Trade Center, began construction in 2006, opening in 2014. Memorials to the attacks include the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.

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