We now know that bin Laden’s terrorists had been planning their outrages for years. The propagation of their mad, bad ideology — decency forbids calling it a religion — had been taking place before our eyes. We were just too blind to see it. In short, the world had never ceased to be dangerous. But the West had ceased to be vigilant. Surely that is the most important lesson of this tragedy, and we must learn it if our civilisation is to survive.