The September 11, 2001 attacks (often referred to as 9/11 ) were a series of coordinated suicide attacks by al-Qaeda upon the United States. On that morning, terrorists affiliated with al-Qaeda hijacked four commercial passenger jet airliners. The hijackers intentionally crashed two of the airliners into the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the building, causing both buildings to collapse within two hours, destroying at least two nearby buildings and damaging others. The hijackers crashed a third airliner into the Pentagon. The fourth plane crashed into a field near Shanksville in rural Somerset County, Pennsylvania after passengers and members of the flight crew on the fourth aircraft attempted to retake control of their plane, which was said to have been heading for The White House. There are no known survivors from any of the flights.

Excluding the 19 hijackers, 2,974 people died in the attacks. Another 24 were missing and presumed dead. The overwhelming majority of casualties were civilians, including nationals from over 90 different countries. In addition, the death of at least one person from lung disease was ruled by a medical examiner to be a result of exposure to dust from the World Trade Center's collapse, as rescue and recovery workers were exposed to airborne contaminants following the World Trade Center's collapse.

The United States responded to the attacks by declaring a War on Terrorism, launching an invasion of Afghanistan to depose the Taliban, who had harbored al-Qaeda terrorists, and enacting the USA PATRIOT Act. Many other nations also strengthened their anti-terrorism legislation and expanded law enforcement powers. Stock exchanges closed for almost a week, and posted enormous losses upon reopening, especially in the airline and insurance industries. The economy of Lower Manhattan ground to a halt, as billions of dollars in office space was damaged or destroyed.

The damage to the Pentagon was cleared and repaired within a year, and a small memorial built on the site. Rebuilding the World Trade Center site has proven more difficult, with controversy over possible designs as well as the pace of construction. The selection of the Freedom Tower for the site has drawn extensive criticism, forcing the abandonment of some parts of the project.