The attack played out in a narrow ninety-eight-minute window. At 8:46, American Airlines Flight 11 struck the North Tower of the World Trade Center; at 9:03, United Airlines 175 hit the South Tower; at 9:37, American 77 struck the Pentagon. United 93 was bound for Washington; when passengers tried to retake the cockpit the aircraft came down in rural Shanksville, Pennsylvania at 10:03. The South Tower collapsed at 9:59, the North Tower at 10:28; the inadequacy of the steel-truss structures under a jet-fuel fire was documented in detail in the 9/11 Commission and NIST reports. 2,977 civilians died (excluding the 19 hijackers): 246 passengers and crew, 2,606 at the World Trade Center, 125 at the Pentagon. Among them were 343 firefighters and 71 police officers.

The organisation was al-Qaeda, led by Osama bin Laden — a jihadist network born from Arab volunteers who had fought the Soviets in Afghanistan in 1988, and relocated to Afghanistan under Taliban rule in 1996. In 1998 bin Laden had issued a fatwa declaring "jihad against Jews and Crusaders"; his stated grievances included the U.S. military presence in Saudi Arabia, the Palestinian issue, and sanctions on Iraq. Fifteen of the nineteen hijackers were Saudi citizens. The operation was prepared over years through cells in Hamburg and Florida, with flight training taken at U.S. schools; total cost is estimated at $400,000–$500,000.

The American response produced a transformation far wider than the attack itself. In October 2001 the invasion of Afghanistan began; the Taliban fell quickly, but the war lasted twenty years and the Taliban returned to Kabul in August 2021. In March 2003 Iraq was invaded — the stated justifications (weapons of mass destruction, an Iraq–al-Qaeda link) were later shown to be unfounded; the vacuum the invasion created set the stage for ISIS in 2014. Domestically: the October 2001 PATRIOT Act expanded electronic surveillance; indefinite detention without trial began at Guantánamo Bay (January 2002, peaking at 780 prisoners); the NSA's mass collection of domestic phone and internet traffic was exposed in 2013 by Edward Snowden's documents. Airport security, border regimes, and visa policies were permanently tightened. By Brown University's Costs of War project, the twenty-year "War on Terror" cost roughly $8 trillion and led to over 900,000 direct and indirect deaths.

The long shadow of 9/11 reaches far beyond security policy. For a generation, the relationship between the Muslim world and the West was read through the frame of that day; migration and Islam debates in Europe, anti-immigrant politics in the U.S., and the global normalisation of the surveillance state all moved on this ground. Two great phenomena of the next twenty years — the rise of populism (Trump in 2016, Brexit) and the strategic ascent of China (after 2008) — are in part the bill for what the United States spent in the Middle East in resources and legitimacy. Each anniversary in New York the Tribute in Light projects two beams into the sky — marking the absence of two towers the city will never get back.