US presidents are often deep into a second term before thoughts turn to legacy. Such is Donald Trump’s elongation of time — a decade’s drama in 18 months — the question feels urgent now. What of Trumpism will last?

Optimists expect the sensible donor class to recapture the Republicans once he has gone. Others suspect he is all too durable: the trigger for an age of populism, not just four or eight years of it. Either way, the president’s tour of Europe has underlined the stakes. He was kinder to Russia than to Nato or the EU. In remarks he later withdrew, Mr Trump doubted US intelligence about foreign meddling in domestic politics. If the future plays out in his shadow, then it promises nothing less than American estrangement from the west.

The mistake is to treat that future as written. A tour that accentuated Mr Trump’s potential as a global spoiler also advertised his probable transience.