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, two of which were flown into the Twin Towers of the World Trade Center in New York City and the third into the Pentagon, which is the headquarters of the U.S. Department of Defense, in Arlington County, Virginia. The fourth plane crashed in a rural Pennsylvania field during a passenger revolt, where the Flight 93 National Memorial was established. In response to the attacks, the United States waged the global war on terror over decades, to eliminate hostile groups deemed terrorist organizations, and the 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. Seventeen minutes later at 9:03 a.m., United Airlines Flight 175 hit the South Tower. Both collapsed within an hour and forty-two minutes, 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 and gave rise to substantial long-term health consequences while also causing at least US$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 crashes of Flight 11 and Flight 175 were the deadliest aviation disasters of all time, and the collision of Flight 77 with the Pentagon resulted in the fourth-highest number of ground fatalities in a plane crash in history. The destruction of the World Trade Center and its environs, located in Manhattan's Financial District, 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.

In 1996, Osama bin Laden of the Islamist militant organization al-Qaeda issued his first fatwā, which declared war against the United States and demanded the expulsion of all American soldiers from the Arabian Peninsula. In a second 1998 fatwā, bin Laden outlined his objections to American foreign policy with respect to Israel, as well as the continued presence of American troops in Saudi Arabia after the Gulf War. Bin Laden maintained that Muslims are obliged to attack American targets until the aggressive policies of the U.S. against Muslims were reversed.

The Hamburg cell in Germany included Islamists who eventually came to be key operatives in the 9/11 attacks. Mohamed Atta; Marwan al-Shehhi; Ziad Jarrah; Ramzi bin al-Shibh; and Said Bahaji were all members of al-Qaeda's Hamburg cell. Bin Laden asserted that all Muslims must wage a defensive war against the United States and combat American aggression. He further argued that military strikes against American assets would send a message to the American people, attempting to force the U.S. to re-evaluate its support to Israel, and other aggressive policies. In a 1998 interview with American journalist John Miller, bin Laden stated:

We do not differentiate between those dressed in military uniforms and civilians; they are all targets in this fatwa. American history does not distinguish between civilians and military, not even women and children. They are the ones who used bombs against Nagasaki. Can these bombs distinguish between infants and military? America does not have a religion that will prevent it from destroying all people. So we tell the Americans as people and we tell the mothers of soldiers and American mothers in general that if they value their lives and the lives of their children, to find a nationalistic government that will look after their interests and not the interests of the Jews. The continuation of tyranny will bring the fight to America, as [the 1993 World Trade Center bomber] Ramzi [Yousef] yourself and others did. This is my message to the American people: to look for a serious government that looks out for their interests and does not attack others, their lands, or their honor. My word to American journalists is not to ask why we did that but to ask what their government has done that forced us to defend ourselves.