The September 11 attacks (also referred to as 9/11)^[a] were a series of four coordinated terrorist attacks by the Islamic terrorist group al-Qaeda^[2][3][4] against the United States on the morning of Tuesday, September 11, 2001. The attacks killed 2,996 people, injured over 6,000 others, and caused at least $10 billion in infrastructure and property damage.^[5][6]Additional people died of 9/11-related cancer and respiratory diseases in the months and years following the attacks.

Four passenger airliners operated by two major U.S. passenger air carriers (United Airlines and American Airlines)\u2014all of which departed from airports in the northeastern part of the United States bound for California\u2014were hijacked by 19 al-Qaeda terrorists. Two of the planes, American Airlines Flight 11and United Airlines Flight 175, were crashed into the North and South towers, respectively, of the World Trade Center complex in Lower Manhattan. Within an hour and 42 minutes, both 110-story towers collapsed. Debris and the resulting fires caused a partial or complete collapse of all other buildings in the World Trade Center complex, including the 47-story 7 World Trade Center tower, as well as significant damage to ten other large surrounding structures. A third plane, American Airlines Flight 77, was crashed into the Pentagon (the headquarters of the United States Department of Defense) in Arlington County, Virginia, which led to a partial collapse of the building's west side. The fourth plane, United Airlines Flight 93, was initially flown toward Washington, D.C., but crashed into a field in Stonycreek Township near Shanksville, Pennsylvania, after its passengers thwarted the hijackers. 9/11 is the single deadliest incident for firefighters and law enforcement officers^[7] in the history of the United States, with 343 and 72 killed, respectively.

Suspicion quickly fell on al-Qaeda. The United States responded by launching the War on Terror and invaded Afghanistan to depose the Taliban, which had failed to comply with U.S. demands to extradite Osama bin Laden and expel al-Qaeda from Afghanistan. Many countries strengthened their anti-terrorism legislation and expanded the powers of law enforcement and intelligence agencies to prevent terrorist attacks. Although al-Qaeda's leader, Osama bin Laden, initially denied any involvement, in 2004 he claimed responsibility for the attacks.^[1] 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, Osama bin Laden was located and killed in Pakistan by SEAL Team Six of the U.S. Navy in May 2011.

The destruction of the World Trade Center and nearby infrastructure seriously harmed the economy of Lower Manhattan and had a significant effect on global markets, which resulted in the closing of Wall Street until September 17 and the civilian airspace in the U.S. and Canada until September 13. Many closings, evacuations, and cancellations followed, out of respect or fear of further attacks. Cleanup of the World Trade Center site was completed in May 2002, and the Pentagon was repaired within a year. On November 18, 2006, construction of One World Trade Center began at the World Trade Center site. The building was officially opened on November 3, 2014.^[8][9] 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 in a field in Stonycreek Township near Shanksville, Pennsylvania.