North Korea is a miserable relic of Stalinist dictatorship, a closed kingdom of starving citizens where military spending runs at about one quarter of gross domestic product, but its leaders have always been good at one thing: international blackmail.

Again and again, they have played on the paranoia of the international community about nuclear proliferation, pressing ahead with uranium enrichment, firing long-range missiles across the Sea of Japan in the guise of satellite launches, and now – for the second time – actually exploding a nuclear device.