Twenty-five years after September 11th 2001, the most consequential question is what has the world learnt from it. The attacks killed nearly 3,000 people while shattering the illusion of American invulnerability and inaugurating two decades of war, radicalisation and instability. After all this, the enduring central lesson remains that terrorism does not disappear because its most visible organisation is defeated. Osama bin Laden’s al-Qaeda did not emerge from out of the blue