The September 11 attacks of 2001 were the deadliest terrorist attacks in human history, causing the deaths of 2,996 people, including 2,977 victims and 19 hijackers who committed murder–suicide. Thousands more were injured, and long-term health effects have arisen as a consequence of the attacks. New York City took the brunt of the death toll when the Twin Towers of the World Trade Center complex in Lower Manhattan were attacked, with an estimated 1,600 victims from the North Tower and around a thousand from the South Tower. Two hundred miles southwest in Arlington County, Virginia, another 125 were killed in the Pentagon. The remaining 265 fatalities included the ninety-two passengers and crew of American Airlines Flight 11, the sixty-five aboard United Airlines Flight 175, the sixty-four on American Airlines Flight 77 and the forty-four who boarded United Airlines Flight 93. The attack on the World Trade Center's North Tower alone made the September 11 attacks the deadliest act of terrorism in human history.

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.