The September 11 attacks, commonly known as 9/11, were four coordinated suicide terrorist attacks carried out by the militant Islamist extremist network al-Qaeda against the United States on September 11, 2001. That morning, nineteen 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, which were two of the top five tallest buildings in the world at the time. Following the first two impacts, the third and fourth flights were similarly coordinated to attack targets in the Washington metropolitan area. The third plane succeeded in crashing into the Pentagon―the headquarters of the U.S. Department of Defense―in Arlington County, Virginia, near the national capital of Washington, D.C. The fourth was intended to strike a federal government building in D.C. itself, but crashed in a Stonycreek, Pennsylvania field following a passenger revolt. The attacks killed nearly 3,000 people and instigated the global war on terror.

The first impact was that of American Airlines Flight 11, which crashed 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, as well as damaging or destroying various other buildings surrounding the towers. A third flight, 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, flew in the direction of Washington, D.C. Alerted to the previous attacks, the plane's passengers attempted to gain control of the aircraft, but the hijackers ultimately crashed the plane in a field in Stonycreek Township, Pennsylvania, near Shanksville at 10:03 a.m. Investigators determined that Flight 93 target was either the United States Capitol or the White House.

Within hours of the attacks, the Central Intelligence Agency determined that al-Qaeda was responsible. The United States formally responded by launching the war on terror and invading Afghanistan to depose the Taliban, which had not complied with U.S. demands to expel al-Qaeda from Afghanistan and extradite its leader, Osama bin Laden. 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 ground forces swept through Afghanistan, bin Laden fled to the White Mountains, where he narrowly avoided capture by U.S.-led forces. Although bin Laden initially denied any involvement, in 2004 he formally claimed responsibility for the attacks. Al-Qaeda's cited motivations included U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq. After evading capture for almost a decade, bin Laden was killed by the U.S. military on May 2, 2011. U.S. and NATO troops remained in Afghanistan until 2021.

The attacks resulted in 2,977 non-hijacker fatalities, an indeterminate number of injuries, and substantial long-term health consequences, in addition to at least $10 billion in infrastructure and property damage. It remains the deadliest terrorist attack in human history as well as the single deadliest incident for firefighters and law enforcement officers in U.S. history, with 340 and 72 killed, respectively. The destruction of the World Trade Center and its environs seriously harmed the New York City economy and induced global market shocks. Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies. 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, the One World Trade Center began construction in November 2006 and 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.