When terrorists crashed hijacked airliners into the World Trade Centre buildings in New York in 2001, their surviving colleagues thought it was a spectacular success. Having both buildings collapse was an unexpected bonus.

More thoughtful heads advised that actually it was a failure, or worse. Though it was traumatic for the citizens of New York and for the friends and relatives of the few thousand people killed, what were a few buildings and a few thousand people to a nation of over 300 million, in which a dozen big buildings are finished every day? The 9/11 attacks did the USA no harm and just made them angry. If you want to kill a snake, strike at the head, don’t just hurt it a bit and make it angry.

But in the longer term, the 9/11 attacks might after all have been a success from a terrorist point of view. The physical damage to the USA at the time was small and repairable, but the anger aroused in the USA was misdirected and self-destructive.

The reckless, impulsive invasion of Iraq was part of that. A year and a half after 9/11 the public mood was still, that someone had to be punished, so that was the pressure on the US administration. So they rushed into Iraq on a flimsy and subsequently discredited excuse, with insufficient troops and no plan for what to do with the place once the short and easy task of toppling the Saddam Hussein regime had been completed.

Seven years, thousands of deaths and trillions of dollars later, the Iraq operation is to be wound down. Osama bin Laden seems still to be alive, and Afghanistan is still not free of the criminal Taliban gang who murder, mutilate and destroy at will. The USA is under a heavy load of debt, its economy is in a long recession and threatened with deflation, and the political system is too polarised and dysfunctional to deal effectively with the nation’s issues.

The USA will perhaps never recover its strength and power and pre-eminent position in the world. One lesson is that if your numbers and resources are small, and you confront a mighty adversary whom you cannot by direct action destroy or even hurt much, then you can get the adversary to use its might self-destructively. Conversely, if you are large and powerful and you are attacked by an entity that seems puny by comparison, your response needs to be carefully thought through, objectively rather than driven by passion and politics.