“If China is not going to solve North Korea, we will.” Like many of Donald Trump’s pronouncements, this sounds like straight talk but leaves crucial questions unanswered. In one aspect, the president’s comments, offered in an interview with the Financial Times, are quite correct. China bears part of the responsibility for the behaviour of its dangerous neighbour and client state. Beijing has long supported the regime in Pyongyang, in part because it provides a barrier between itself and US-friendly South Korea and the American troops posted there.

China also has leverage. It provides indispensable supplies of food and fuel to the hermit kingdom. No other country has remotely comparable economic influence. Beijing is concerned that amplifying the pressure could backfire, inciting even more unpredictable behaviour; but the depth of the crisis demands that Mr Xi start to use some of his diplomatic capital.

Mr Trump is on much weaker ground when he suggests that, if China fails to live up to its responsibilities, the US can solve the problem itself. Perhaps his comments are intended to raise the prospect of a military option. Unilateral armed intervention would however be calamitously risky. If North Korea’s nuclear capacities were taken out with bunker-busting missile strikes, Pyongyang could destroy Seoul with its artillery almost immediately. Its missiles could reach Japan. Millions of lives would be at stake.