The al-Qaeda attacks of September 11, 2001 — four hijacked airliners, the collapse of the World Trade Center's twin towers, a strike on the Pentagon, and a fourth plane forced down in Pennsylvania — killing roughly 2,977 people and triggering two decades of American-led wars, the construction of a domestic surveillance and security state, and a global transformation of how governments justified coercion in the name of safety.
At 8:46 on a clear Tuesday morning, American Airlines Flight 11 flew into the North Tower of the World Trade Center. Most people who saw the first images assumed an accident. Seventeen minutes later, the second plane hit the South Tower, and the assumption dissolved. By 10:28, both towers were gone. Before that morning, the United States had spent the decade since the Cold War without a recognized enemy of comparable weight. After it, the country would spend the next twenty years at war — and would remake its own laws, institutions, and global relationships to fight an adversary that wore no uniform and held no territory. The question September 11 forced onto the world was not only who had done this, but what a superpower could legitimately do in response. That question has never been settled.