The National Commission on Terrorist Attacks Upon the United States, commonly known as the 9/11 Commission, was set up on November 27, 2002, to investigate all aspects of the September 11 attacks, the deadliest terrorist attack in world history. It was created by Congressional legislation, which charged it with preparing "a full and complete account of the circumstances surrounding the September 11 attacks", including preparedness by the U.S. federal government for the attacks, the response following the attacks, and steps that can be taken to guard against a future terrorist attack.

The September 11 attacks, commonly known as 9/11, were four coordinated Islamist suicide terrorist attacks carried out by Al-Qaeda against the United States on September 11, 2001. That morning, 19 terrorists hijacked four commercial airliners scheduled to travel from the East Coast to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City, two of the world's five tallest buildings at the time, and aimed the next two flights toward targets in or near Washington, D.C., in an attack on the nation's capital. The third team succeeded in striking the Pentagon, the headquarters of the U.S. Department of Defense in Arlington County, Virginia, while the fourth plane crashed in rural Pennsylvania during a passenger revolt. The September 11 attacks killed 2,977 people, making them the deadliest terrorist attack in history, and instigated the multi-decade global war on terror, fought in Afghanistan, Iraq, and elsewhere.