Diplomacy is about getting other countries to do what you want them to do. Without China’s help, Donald Trump cannot hope to enforce his deal with Kim Jong Un. Eliminating North Korea’s nuclear arsenal is Mr Trump’s biggest goal — his reputation for dealmaking rests almost exclusively on it. Yet reaching that prize undercuts a key element of his America First agenda. On Friday, his administration will unveil the latest list of targeted Chinese goods. A trade war with China could prove disastrous to the North Korea agreement. Mr Trump is thus faced with a choice. Should he wreck the deal of the century by escalating America’s trade dispute with China? Or should he confine his trade belligerence to America’s allies?

It says a lot about Mr Trump’s volatility that few have much clue which path he will take. The first choice — picking a fight with China when he most needs its help — would vindicate those who think Mr Trump is irrational. It would be like putting his money on a horse then slipping arsenic into its oats. China would lift its sanctions on North Korea. Since 90 per cent of North Korea’s trade is with China, that would end its isolation. Without China, there would have been no “maximum pressure”. Without Xi Jinping’s prodding, Mr Kim may not have even agreed to this week’s summit.

Such a move would confirm the “diplotainment” theory of Mr Trump’s motives. Rather than caring about outcomes, Mr Trump lives for day-to-day ratings. His meeting with Kim Jong Un was the epitome of reality TV diplomacy. Simply by holding the summit, his prize was achieved. The celebrity leaders met and made diplomatic history. On his return from Singapore, Mr Trump tweeted: “There is no longer a Nuclear Threat from North Korea.” If Mr Trump really believed that, it would be hard to retain much hope that he is rational. North Korea has just the same nuclear capacity after the summit as it had beforehand. Evidence of Mr Trump’s inability to fix means to ends would come if he chose now to step up his trade war with China. As the Chinese could tell him, a man who chases two rabbits catches neither.