The September 11 attacks, commonly known as 9/11, were four coordinated suicide terrorist attacks carried out by the militant Islamic extremist network al-Qaeda against the United States on Tuesday, September 11, 2001. That morning, nineteen terrorists hijacked four commercial airliners scheduled to travel from the northeastern U.S. to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City, and the third plane into the Pentagon (the headquarters of the American military) in Arlington, Virginia. The fourth plane was intended to hit a federal government building in Washington, D.C., but crashed in a 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 at 8:46 am, into the North Tower of the World Trade Center complex in Lower Manhattan. 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, precipitating the collapse of other World Trade Center structures including 7 World Trade Center, and damaging nearby buildings. A third flight, American Airlines Flight 77, crashed into the west side of the Pentagon in Arlington County, Virginia, at 9:37 am, causing a partial collapse. The fourth and final flight, United Airlines Flight 93, flew in the direction of Washington, D.C. Alerted of the previous attacks, the plane's passengers attempted to regain control, but the hijackers ultimately crashed the plane in a field in Stonycreek Township, Pennsylvania, near Shanksville, at 10:03 am. Investigators determined that Flight 93 was targeting either the U.S. Capitol or the White House.

Within hours of the attacks, suspicion quickly fell onto al-Qaeda. 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 NATOits only usage to datecalled 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.

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 and 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.

The original World Trade Center (WTC) was a large complex of seven buildings in the Financial District of Lower Manhattan, New York City, United States. It opened on April 4, 1973, and was destroyed in 2001 during the September 11 attacks. At the time of their completion, the Twin Towers—the original 1 World Trade Center (the North Tower) at 1,368 feet (417 m); and 2 World Trade Center (the South Tower) at 1,362 feet (415.1 m)—were the tallest buildings in the world. Other buildings in the complex included the Marriott World Trade Center (3 WTC), 4 WTC, 5 WTC, 6 WTC, and 7 WTC. The complex contained 13,400,000 square feet (1,240,000 m2) of office space.

The core complex was built between 1966 and 1975, at a cost of $400 million (equivalent to $2.27 billion in 2021). The idea was suggested by David Rockefeller to help stimulate urban renewal in Lower Manhattan, and his brother Nelson signed the legislation to build it. The buildings at the complex were designed by Minoru Yamasaki. In 1998, the Port Authority of New York and New Jersey decided to privatize it by leasing the buildings to a private company to manage. It awarded the lease to Silverstein Properties in July 2001. During its existence, the World Trade Center symbolized globalization and the economic power of America. Although its design was initially criticized by New York citizens and professional critics, the Twin Towers became an icon of New York City. It had a major role in popular culture, and according to one estimate was depicted in 472 films. The Twin Towers were also used in Philippe Petit's frequent tightrope-walking performance on August 7, 1974. Following the September 11 attacks, mentions of the complex in various media were altered or deleted, and several dozen "memorial films" were created.The World Trade Center experienced several major crime and terrorist incidents, including a fire on February 13, 1975; a bombing on February 26, 1993; a bank robbery on January 14, 1998, and finally a terrorist attack on September 11, 2001. The latter began after Al-Qaeda-affiliated hijackers flew two Boeing 767 jets into the Twin Towers within minutes of each other. Between 16,400 and 18,000 people were in the Twin Towers when they were struck. The fires from the impacts were intensified by the planes’ burning jet fuel, which along with the initial damage to the buildings’ structural columns, ultimately caused both towers to collapse. The attacks in New York City killed 2,606 people in and within the vicinity of the towers, as well as all 157 on board the two aircraft. Falling debris from the towers, combined with fires that the debris initiated in several surrounding buildings, led to the partial or complete collapse of all the WTC complex's buildings including 7 World Trade Center, and caused catastrophic damage to 10 other large structures in the surrounding area.

The cleanup and recovery process at the World Trade Center site took eight months, during which the remains of the other buildings were demolished. On May 30, 2002, the last piece of WTC steel was ceremonially removed. A new World Trade Center complex is being built with six new skyscrapers and several other buildings, many of which are complete. A memorial and museum to those killed in the attacks, a new rapid transit hub, and an elevated park have been opened. The memorial features two square reflecting pools in the center marking where the Twin Towers stood. One World Trade Center, the tallest building in the Western Hemisphere at 1,776 feet (541 m) and the lead building for the new complex, was completed in May 2013 and opened in November 2014.