It was that event that set in motion the rapid collapse of one of the world's two superpowers. It was achieved not by the United States and its military might, but by a small group of religiously inspired fighters, the mujahideen and their helpers. Ibn Khaldun's theory was that every urban civilisation becomes vulnerable when it grows decadent from within. People live in towns and get used to luxuries. The rich grow indolent, the poor resentful. There is a loss of asabiyah, a keyword for Khaldun. Nowadays we would probably translate it as "social cohesion". People no longer think in terms of the common good. They are no longer willing to make sacrifices for one another. Essentially they lose the will to defend themselves. They then become easy prey for the desert dwellers, the people used to fighting to stay alive.

That, so it seemed to those who read history that way, is what happened in Afghanistan. It was never possible for a small group to defeat a superpower by conventional means. But it could go on endlessly inflicting casualty after casualty until eventually the superpower — more like a lumbering elephant than a wounded lion — withdrew. The desert dwellers are hungrier, tougher and more ruthless than the city dwellers who long more than anything for a quiet life.

That was the calculation. The odd thing is, it worked. And those who had fought the Soviet Union looked on in wonder at the effect of their victory. For not only did the Russians withdraw. Within an extraordinarily short time their whole empire collapsed. Ibn Khaldun was right. The society had grown rotten from within. It had lost its asabiyah, its cohesion. It had lost the will to fight.

If that is what a small group of highly motivated religious fighters could do to one superpower, why not the other, America and the West? America could not be defeated on its own ground. But what if it could be tempted, provoked, into occupying the very same ground that had seen the humiliating withdrawal of the Soviet army, namely Afghanistan itself? To do so would require a truly massive provocation, one so shocking that it would make the Americans forget what everyone knew, that Afghanistan is a death trap that ultimately defeats all invading armies. That is when 9/11 was born.

The theory was that the Americans and the Russians might be unalike in every other respect, but this they shared: that they were advanced urban civilisations in which the social bond, asabiyah, had grown weak. They were no longer lean and hungry. They were overweight and lacked the capacity for sustained sacrifice. If America could be provoked into occupying Afghanistan, it could be defeated exactly as the Soviets had been, not by any decisive battle but by sustained asymmetric warfare. The proof was that American troops had withdrawn from Lebanon in 1984 and Somalia in 1994 under just such circumstances. They had no more staying power than the Russians. Like the Russians, within a decade they would be looking for an exit strategy. 9/11 was the attempt to lure the United States into Afghanistan, and it worked.