People facing alarming birthdays often say things like: “Forty is just a number.” You could say the same about “$100 oil”. But such benchmarks concentrate minds. As the oil price threatens to break through $100, politicians all over the world will think hard about the strategic consequences.

So what is likely to happen? The biggest single effect is obvious. Oil producers become richer and more powerful. The biggest oil consumers – the US, China and the European Union – become increasingly anxious. Beneath that big trend, there are smaller effects that could change the course of some of the most delicate and dangerous problems – Iraq, Iran, China’s foreign policy and the resurgence of Russia.