September 11 attacks
The September 11 attacks, commonly known as 9/11, were four coordinated Islamist terrorist suicide attacks carried out by al-Qaeda against the United States in 2001. On that morning, 19 terrorists hijacked four commercial airliners scheduled to travel from the East Coast to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City and aimed the next two flights toward targets in or near Washington, D.C., in an attack on the nation's capital. The third team succeeded in striking the Pentagon, the headquarters of the U.S. Department of Defense in Arlington County, Virginia, while the fourth plane crashed in rural Pennsylvania 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, in Afghanistan, Iraq, Syria, and several other countries.

Ringleader Mohamed Atta flew American Airlines Flight 11 into the North Tower of the World Trade Center complex in Lower Manhattan at 8:46 a.m. Seventeen minutes later, at 9:03, the World Trade Center's South Tower was hit by United Airlines Flight 175. Both 110-story skyscrapers collapsed within an hour and forty-one minutes, bringing about the destruction of the remaining five structures in the WTC complex and damaging or destroying nearby buildings. American Airlines Flight 77 flew towards Washington, D.C. and crashed into the Pentagon at 9:37 a.m., causing a partial collapse. The fourth and final flight, United Airlines Flight 93, also changed course towards Washington, 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 then crashed the aircraft into a Stonycreek Township field, near Shanksville, Pennsylvania, at 10:03 am. The Federal Aviation Administration ordered an indefinite ground stop for all air traffic in U.S. airspace at 9:45 am (59 minutes following the first impact), 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 formally responded by launching the war on terror and invading Afghanistan to depose the Taliban, which rejected the conditions of U.S. terms to expel Al-Qaeda from Afghanistan and extradite its leaders. The U.S.'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 NATO invasion forces swept through Afghanistan, bin Laden eluded them by disappearing into the White Mountains. 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 on May 2, 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 last members of the U.S. armed forces left the region on August 30, 2021, after which the Taliban rapidly returned to power. Ayman al-Zawahiri, another planner of the attacks who succeeded bin Laden as leader of Al-Qaeda, was killed by U.S. drone strikes in Kabul, Afghanistan on July 31, 2022.

Excluding the hijackers, the attacks killed 2,977 people, injured thousands more 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") took eight months and 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 November 2006; it opened in November 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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