This is the gravest crisis for Western security since the end of World War Two, and a lasting one. As one expert puts it, “Trumpism will outlast his presidency”. But which nations are equipped to step to the fore as the US stands back?

At 09.00 one morning in February 1947, the UK ambassador in Washington, Lord Inverchapel, walked into the State Department to hand the US Secretary of State, George Marshall, two diplomatic messages printed on blue paper to emphasise their importance: one on Greece, the other on Turkey.

Exhausted, broke and heavily in debt to the United States, Britain told the US that it could no longer continue its support for the Greek government forces that were fighting an armed Communist insurgency. Britain had already announced plans to pull out of Palestine and India and to wind down its presence in Egypt.

The United States saw immediately that there was now a real danger that Greece would fall to the Communists and, by extension, to Soviet control. And if Greece went, the United States feared that Turkey could be next, giving Moscow control of the Eastern Mediterranean including, potentially, the Suez Canal, a vital global trade route.

Almost overnight, the United States stepped into the vacuum left by the departing British.

“It must be a policy of the United States,” President Harry Truman announced, “to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressure.”

It was the start of what became known as the Truman Doctrine. At its heart was the idea that helping to defend democracy abroad was vital to the United States’ national interests.

There followed two major US initiatives: the Marshall Plan, a massive package of assistance to rebuild the shattered economies of Europe, and the creation of Nato in 1949, which was designed to defend democracies from a Soviet Union that had now extended its control over the eastern part of Europe.

It is easy to see this as the moment that leadership of the western world passed from Britain to the United States. More accurately it is the moment that revealed that it already had.

The United States, traditionally isolationist and safely sheltered by two vast oceans, had emerged from World War Two as the leader of the free world. As America projected its power around the globe, it spent the post-war decades remaking much of the world in its own image.

The baby boomer generation grew up in a world that looked, sounded and behaved more like the United States than ever before. And it became the western world’s cultural, economic and military hegemon.

Yet the fundamental assumptions on which the United States has based its geostrategic ambitions now look set to change.

Donald Trump is the first US President since World War Two to challenge the role that his country set for itself many decades ago. And he is doing this in such a way that, to many, the old world order appears to be over – and the new world order has yet to take shape.

The question is, which nations will step forward? And, with the security of Europe under greater strain than at any time almost in living memory, can its leaders, who are currently scrabbling around, find an adequate response?

President Trump’s critique of the post-1945 international order dates back decades. Nearly 40 years ago he took out full-page advertisements in three US newspapers to criticise the United States’ commitment to the defence of the world’s democracies.

“For decades, Japan and other nations have been taking advantage of the United States,” he wrote in 1987. “Why are these nations not paying the United States for the human lives and billions of dollars we are losing to protect their interests?

“The world is laughing at America’s politicians as we protect ships we don’t own, carrying oil we don’t need, destined for allies who won’t help.”

It’s a position he has repeated since his second inauguration.

And the fury felt by some in his administration for what they perceive as European reliance on the United States was apparently shown in the leaked messages about air strikes on Houthis in Yemen that emerged this week.

In the messages, an account named Vice-President JD Vance wrote that European countries might benefit from the strikes. It said: “I just hate bailing Europe out again.”

Another account, identified as Defence Secretary Pete Hegseth, responded three minutes later: “VP: I fully share your loathing of European free-loading. It’s PATHETIC.”

Trump’s own position appears to go beyond criticising those he says are taking advantage of the United State’s generosity. At the start of his second presidency, he seemed to embrace Russian President Vladimir Putin, telling Russia that Ukraine would not be granted Nato membership and that it should not expect to get back the territory it has lost to Russia.

Many saw this as giving away two major bargaining chips before talks had even started. He apparently asked Russia for nothing in return.

On the flipside, certain Trump supporters see in Putin a strong leader who embodies many of the conservative values they themselves share.

To some, Putin is an ally in a “war on woke”.

The United States’ foreign policy is now driven, in part at least, by the imperatives of it