Why does China tolerate North Korea’s extraordinary behaviour? The relationship is an unusual one – not based on mutual respect and friendship but on cold calculation. Beijing has agreed to support US moves at the UN to place sanctions on North Korea, but the world is puzzled by its apparent reluctance to follow Washington’s lead with any great enthusiasm.

China was supportive of North Korea during the Korean war (1950-53). The Chinese may also see Kim Jong-eun’s nuclear posturing as a rational use of the extremely weak cards in his hand. After all, the regime presides over tens of millions of malnourished and miserable people and it is desperate not to fall. But China and North Korea are not happy neighbours.

The two countries disagree about their border; China occupies part of Baekdu-san, a mountain seen as sacred within Korea. Both countries believe that parts of the Tumen river delta are wrongly occupied by the other. Last year, uniformed North Korean sailors kidnapped, beat and robbed 28 Chinese fishermen in Chinese waters. It cannot help that North Korea has a racist view about the superiority of their own people – interracial liaisons can lead to forced abortions and imprisonment.