Normally, personal relationships between elected leaders scarcely matter. Most leaders follow the briefing books written by their bureaucracy. They pursue long-established national policies. Any talk of personal “chemistry” is usually just gossip.

But it’s different with Donald Trump. After a career running a small family company without a board, he doesn’t do bureaucracy. He surrounds himself with trusted lackeys. Anyone who manages to get a word in his ear therefore has a good chance of shaping the world’s fate.

So far, Trump hasn’t bothered much with foreign policy. Moving the US embassy to Jerusalem and leaving the Paris climate accords were chiefly symbolic acts. But that could soon change. His tax bill may prove his last big domestic legislation, especially given his shrunken Senate majority. That will free up more head space to get worked up about foreign issues, from North Korea to Iran. Other western countries therefore face an urgent question — how to influence this man?