On the morning of 11 September 2001, 19 al-Qaeda terrorists hijacked four commercial aircraft in a coordinated attack on the United States. Two planes were flown into the Twin Towers of the World Trade Center in New York City, which collapsed within two hours; one struck the Pentagon in Arlington, Virginia; and the fourth — United Airlines Flight 93, whose passengers stormed the cockpit — crashed in a field near Shanksville, Pennsylvania, likely aimed at the US Capitol.
Nearly 3,000 people were killed — the deadliest terrorist attack in history. The images of the towers burning and falling were watched live by hundreds of millions worldwide.
The attacks triggered the US invasion of Afghanistan (October 2001), aimed at destroying al-Qaeda and removing the Taliban government that harboured them; and the invasion of Iraq (2003). They reshaped global security infrastructure, intelligence sharing, and civil liberties — giving rise to the NSA surveillance programmes later revealed by Edward Snowden. Two decades of the "War on Terror" that followed cost over $8 trillion and claimed hundreds of thousands of lives across multiple countries.