At 8.46am on a bright autumn morning 25 years ago, American Airlines Flight 11 slammed into the upper floors of the North Tower of the World Trade Center in New York. Seventeen minutes later, United Airlines Flight 175 hit the South Tower. Within two hours both had collapsed, leaving nearly 3,000 dead. Another hijacked plane was flown into the Pentagon building in Washington DC, while the final flight crashed into a Pennsylvania field.A quarter of a century on, 9/11 still casts a long, complicated shadow. For the thousands of bereaved families who will mourn again today at the site of the atrocity in lower Manhattan, the grief remains raw. For the hundreds of millions who watched it unfold on their screens, the attacks stand alongside Dallas in 1963 or Berlin in 1989 as a hinge moment in history, imprinted forever on all who witnessed it.In the immediate aftermath, many insisted the world had changed forever. That claim still holds, though it needs heavy qualification. In the popular imagination, 9/11 marks the end of post-Cold War optimism and the dawn of a more fraught and insecure era. In reality, history is never so neat. But the consequences for millions of people were real enough.The attacks set in train a disastrous chain of events. A cabal of ideologues in the Bush administration saw an opportunity to test their theories of how military power might reshape the world to their liking. The result was a catastrophic adventure in the Middle East that left hundreds of thousands dead and spawned, in Islamic State, an enemy even more ruthless than al-Qaeda, the group behind the attacks. That was a vindication of Osama bin Laden’s calculation that an extreme provocation would invite a disproportionate response, recruiting more followers to the cause.Bin Laden was killed by US special forces in 2011, but his malevolent legacy can be seen in the conspiracy theories and bitter cynicism that now blight public discourse well beyond the US, and the rise of virulent new strains of nativism and xenophobia across the world.The 9/11 attacks were both nihilistic and terrifyingly inventive. A handful of young fanatics, armed with box cutters and rudimentary flight training, exploited a fatal set of national security vulnerabilities. The spectacle they manufactured was tailor-made for the still-new medium of rolling television news and seemed to deliberately parody the apocalyptic imagery of America’s own entertainment industry.But as 9/11 recedes gradually from living memory into history, what must not be forgotten is the brutal cutting short of so many lives on the day and in what followed. Humanity’s capacity to inflict death and misery in the name of an idea remains undimmed.