The September 11 attacks, also commonly referred to as 9/11,[a] were a series of four coordinated terrorist attacks by the militant Islamist terrorist group al-Qaeda[3][4][5] against the United States on Tuesday, September 11, 2001. On that morning, four commercial airliners traveling from the northeastern U.S. to California were hijacked mid-flight by 19 al-Qaeda terrorists. The hijackers were organized into three groups of five members and one group of four. Each group had one hijacker who had received flight training and took over control of the aircraft. Their explicit goal was to crash each plane into a prominent American building, causing mass casualties and partial or complete destruction of the targeted buildings. Two of the planes hit the Twin Towers of the World Trade Center, and a third hit the west side of the Pentagon. A fourth plane was intended to crash into a target in Washington, D.C., but instead crashed into a field near Shanksville, Pennsylvania, following a passenger revolt.[6]

The first plane to hit its target was American Airlines Flight 11. It was flown into the North Tower of the World Trade Center complex in Lower Manhattan at 8:46 am. Seventeen minutes later at 9:03 am, the World Trade Center's South Tower was hit by United Airlines Flight 175. Both 110-story towers collapsed within an hour and forty-two minutes, leading to the collapse of the other World Trade Center structures including 7 World Trade Center, and significantly damaging surrounding buildings. A third hijacked flight, American Airlines Flight 77, crashed into the west side of the Pentagon (the headquarters of the American military) in Arlington County, Virginia at 9:37 am, causing a partial collapse of the building's side. The fourth, and final flight, United Airlines Flight 93, was flown in the direction of Washington, D.C. The plane's passengers attempted to regain control of the aircraft away from the hijackers and ultimately diverted the flight from its intended target; it crashed into a field near Shanksville, Pennsylvania, at 10:03 am. Investigators determined that Flight 93's target was either the U.S. Capitol or the White House.

In the immediate aftermath of the attacks, suspicion quickly fell onto al-Qaeda. The United States government under the George W. Bush administration 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 al-Qaeda leader Osama bin Laden. Bin Laden fled to the White Mountains where he came under attack by U.S.-led forces, but managed to escape.[7] Although bin Laden initially denied any involvement, in 2004 he formally claimed responsibility for the attacks.[2] Al-Qaeda and bin Laden cited U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq as motives. After evading capture for almost a decade, bin Laden was located in a hideout in Abbottabad, Pakistan, and subsequently killed by the U.S. military on May 2, 2011.

The destruction of the World Trade Center and nearby infrastructure seriously harmed the economy of New York City and created a global economic recession. Many countries strengthened their anti-terrorism legislation and expanded the powers of law enforcement and intelligence agencies to prevent terrorist attacks. The U.S. and Canadian civilian airspaces were closed until September 13, while Wall Street trading was closed until September 17. Many closings, evacuations, and cancellations followed, out of respect or fear of further attacks. Cleanup of the World Trade Center site took eight months and was completed in May 2002, while the Pentagon was repaired within a year. Design of a replacement World Trade Center complex took several years because of the many stakeholders involved. Work on the new iconic building for the site, One World Trade Center, began in November 2006, and opened in November 2014 after several construction delays.[8][9]

The attacks resulted in 2,977 fatalities, over 25,000 injuries, and substantial long-term health consequences, in addition to at least $10 billion in infrastructure and property damage.[10][11] It remains the deadliest terrorist attack in human history and the single deadliest incident for firefighters and law enforcement officers in the history of the United States, with 340[12] and 72 killed,[13][14] respectively. Numerous memorials have been constructed, including 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.