The rage in the White House is unbounded. North Korean leader Kim Jong Un — “Little Rocket Man”, the US president calls him — must be destroyed. The international nuclear agreement with Iran is the worst deal ever. Free trade is a conspiracy against the US. America’s allies are freeloaders. It is a struggle not to conclude the real and present danger to international peace and security now sits at the point of collision between Donald Trump’s narcissism and the limits on US power.

As a candidate, Mr Trump promised to bury liberal internationalism. He would throw off global entanglements in favour of America-first nationalism. As president, he now wants the world to do as he tells, or tweets, it. Mr Trump is unaccustomed to defiance, especially from those with foreign-sounding names from unfamiliar places on the map. In threatening to eviscerate Pyongyang or disavowing the nuclear accord with Tehran, the president is nothing so much as an angry ego confounded by the failure to get his own way.

The outbursts have consequences, something I was reminded of during a few days this week in Seoul. The drums of war beat more ominously when you are within easy range of North Korea’s artillery batteries. Not so much because South Koreans live in permanent fear. These are stoics grown accustomed to the threat from the north. More because, in Mr Kim, Pyongyang has a leader as volatile as the US president. The rules of containment, deterrence and the rest depend on a certain predictability on both sides.