Churchill was not naive. For the previous 20 years he had been the most vociferous opponent of the Soviet Union and all it stood for. He was well aware that when the alliance with Stalin led to the defeat of Hitler’s Germany the Soviet Union would emerge far more powerful than before and control much of eastern Europe.

But history has not found fault in the judgment that the price was worth paying if Britain was to be freed from Nazi invasion and western Europe from Hitler’s domination. The balance of advantage and disadvantage of working with the Iranians is not nearly as difficult as it was with the Russians. Iran, unlike the Soviet Union, will never be a superpower or a global threat.

Its economy is in a mess and the future of the ayatollah’s regime is far from certain. If the co-operation of the Iranians in keeping Iraq together and defeating the jihadi terrorists would suit Tehran, the reality is that it would help the moderate Arab governments and the West even more.

An Islamic State straddling Iraq and Syria would destabilise the whole Arab world and be a base for exporting terrorism not just in the Middle East but throughout Europe and the United States. Furthermore, it would accelerate the radicalisation of a small but dangerous number of Muslims throughout the world.