The question worrying Foreign Office mandarins is whether Mr Trump’s tenure in the White House represents a temporary blip, with normal service resumed once he’s replaced by someone saner, or whether we’re witnessing a long-term shift in the way the US deals with the rest of the world – including its former friends.

Sir Keir Starmer has done his best to keep the “special relationship” alive. He ensured the UK pulled out all the stops when Mr Trump enjoyed a glamorous second state visit to Britain last September, hosted by the King. Around 120 horses and 1,300 members of the British military were involved in ceremonies at Windsor Castle.

The Prime Minister made the message explicit in his speech welcoming the President, saying: “This relationship is not just about history. It’s about the future.”

He had the right idea. Partnership with the US is the foundation of the UK’s defence and foreign policy, and America is our biggest trading partner (unless we count the entire EU as a single partner).

But it hasn’t worked, as we saw this week when Donald Trump threw an extraordinary tantrum in response to plans for the UK, and European allies, to defend the region known as the High North. This includes Greenland, part of the Kingdom of Denmark, which Mr Trump wants to buy.

The President announced additional tariffs on goods shipped to the US from nations including the UK. He also attacked Sir Keir over plans to hand sovereignty of the Chagos Islands to Mauritius, calling it “an act of GREAT STUPIDITY”.

Mr Trump regards both tariffs and public criticism as a method of punishing those who cross him. It suggests that all Sir Keir’s diplomacy was for naught, because while allies might sometimes disagree, they never behave like this.

Bizarrely, the President’s outburst appears to have been prompted by a failure to understand the message from the UK and European partners. They wanted to reassure the White House that they take concerns about Greenland’s security seriously, and are willing to help.

Instead, Mr Trump took it to mean that Europe was threatening military action against the US. The fact that the President could misread the signals so severely suggests he is failing to listen to foreign policy experts in his administration.

In the end, the tariff threat was lifted – but the President then chose to insult the UK, and other NATO allies, by suggesting our armed forces failed to contribute to the war against the Taliban in Afghanistan. The reality is that 457 British personnel died.

With any luck, the current chaos in the White House will go away once Mr Trump is replaced by a new president in 2029. But that’s an issue of competence, not political ideology. And we appear to be witnessing a long-term change in the way America deals with the rest of the world.

There’s a method to Donald Trump’s madness. He’s not just motivated by ego – there’s also a coherent political philosophy which is shared by others in Washington.

The President believes, of course, that America must be “strong and respected”. But he also believes there should be a limit to America’s ambitions. No longer should the US aspire to be the world’s policeman. Instead, it should aim to dominate the “Western Hemisphere”, by which he basically means the continents of North and South America. Hence, the US used its military might to intervene in Venezuela earlier this month.

And the rest of the world is on its own.

This was spelt out in the National Security Strategy published by the White House last November. It warned: “After the end of the Cold War, American foreign policy elites convinced themselves that permanent American domination of the entire world was in the best interests of our country. Yet the affairs of other countries are our concern only if their activities directly threaten our interests.”

Explaining his apparent lack of concern about Russia’s invasion of Ukraine, Mr Trump explained last year: “We have a big, beautiful ocean as a separation”. It was a typically tactless comment, but one that accurately summed up the new US approach to world affairs.

And it’s an approach many Republicans welcome, regardless of their feelings about Mr Trump.

Stung by the huge cost in American lives and dollars of military interventions across the world, they believe it is time to collect the long-awaited “peace dividend” they were promised when the Cold War ended with the defeat of Soviet Communism, 20-odd years ago.

There is also a political divide. The Security Strategy paints a sorry picture of Europe as a continent afflicted by economic decline and “civilizational erasure”, characterised by falling birthrates and loss of self-confidence, although it suggests we could become “great” again if “patriotic European parties” gain power.