The Ahmadinejads of the world could always operate in certainty their tirades could be launched at Washington with relative impunity. In extreme cases the White House might order a targeted strike or two at an airbase or missile site. Barack Obama’s credibility took an enormous hit when he drew a “red line” for Syria, then failed to enforce it. Russian President Vladimir Putin took the measure of the man and proceeded to exploit it ruthlessly.

Trump’s approach may be less civil, but it succeeds in the same way as his overall method of governing: by sowing chaos and confusion, and projecting a constant state of uncertainty, he keeps opponents off balance. One reason Ronald Reagan was a success at staring down the Soviet Union was because Moscow was never quite sure whether he was crazy enough to start a war. Critics thought his space-based missile project was a mad dream, but the possibility he might pursue it prompted caution all-round. Don’t get the guy worked up — you never know what might happen.

Trump is many times less predictable than Reagan, and thus harder to handle for foreign powers. China has learned that, as Trump has happily jacked up tariff after tariff in pursuit of a trade deal, regardless of the moaning of economists, farmers and allies, and the damaging impact on international trade. The pain has been enough to produce a one-on-one meeting with President Xi Jinping at the G20 summit in Japan, which Trump is entering with a threat to “take in billions and billions of dollars a month and … do less and less business with them,” if things don’t go his way.