On September 11, 2001, 19 members of the Islamic terrorist group Al-Qaeda hijacked four commercial airliners and utilized them in a suicide attack on American soil. At 8:46 a.m., American Airlines Flight 11 crashed into the North Tower of the World Trade Center in New York City. The South Tower was struck by United Airlines Flight 175 shortly afterwards at 9:03 a.m. The third aircraft, American Airlines Flight 77, struck the Pentagon in Arlington, Virginia, at 9:37 a.m. A fourth aircraft, United Airlines Flight 93, crashed into a field near Shanksville, Pennsylvania, at 10:03 a.m. when the passengers and crew fought back against the hijackers to prevent a further attack (it is believed that the hijackers' intended target for this flight was either the White House or the US Capitol Building). The ensuing fires in the North and South Towers trapped people in floors above the impact zone. Combined with the damage inflicted in the initial impact, the heat of the fires further weakened the structures of the buildings, leading to their collapse. At 9:59 a.m., the South Tower collapsed. The North Tower collapsed as well at almost a half hour later at 10:28 a.m. The debris from the collapsing towers damaged and set fire to the nearby 7 World Trade Center building, leading to its collapse at 5:21 p.m.

2,977 men, women, and children were killed in the attacks. Among the victims were 343 firefighters and 72 law enforcement officers who had selflessly rushed into the buildings to rescue as many people as they could. Furthermore, 55 American military personnel were killed in the attacks, including Lieutenant General Timothy Maude, a Deputy Chief of Staff and the highest-ranking military official killed at the Pentagon that day. The youngest victim of the attack was only two and a half years old.

In the immediate aftermath of the attack, all air traffic in the United States was ground for several days. On September 14, Congress passed the Authorization of Use of Military Force joint resolution allowing the President to use force against those who had "planned, authorized, committed, or aided" the September 11 attacks or those who harbored them. This resolution remains in effect to this day. Furthermore, the US invoked Article V of the North Atlantic Treaty, calling for all members of the North Atlantic Treaty Organization (NATO) to come to their aid. On October 4, the membership of NATO confirmed the invocation, marking the only time in the organization's history that Article V had been successfully invoked. Additionally, the United States, supported by numerous allies, launched the invasion of Afghanistan on October 7 following the Taliban's refusal to extradite Osama bin Laden (leader of Al-Qaeda) to the United States. On May 2, 2011, bin Laden was finally killed by US Navy SEALs during a raid on his compound in neighboring Pakistan by forces of the Joint Special Operations Command, including SEALs, the US Army's 160th Special Operations Aviation Regiment (SOAR) and the Central Intelligence Agency's Special Activities Division (CIA-SAD). The War in Afghanistan, however, continues to this day.