The thing about MAD is that it requires both sides to be sane. Ever since the onset of the nuclear age, the doctrine of mutually assured destruction, or MAD, has kept the peace. The calculation that, ultimately, no rational political leadership would risk millions of deaths in their own nation has seen the world through some perilous moments – from the Cuba missile crisis to the fall of the Berlin Wall.

The most alarming aspect of the current crisis with a nuclear-armed North Korea is that the regime there might be one of those rare abberations, to which the normal logic of nuclear deterrence does not apply.

Every now and then, during the cold war, there was a suggestion that some political leader might be prepared to think about the supposedly unthinkable. In the late 1950s, Mao Zedong shocked even the hard-bitten former Stalinists of the Soviet Union when, on a trip to Moscow, he suggested that nuclear war might not be so bad after all, telling his startled hosts: “If the worst came to the worst and half of mankind died, the other half would remain. Imperialism would be destroyed, and the whole world would become socialist.”