
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.

President Truman said the United States must support free nations
"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 America's global role
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?
A challenge to the Truman legacy
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."

In leaked messages, an account named Vice President JD Vance wrote: "I just hate bailing Europe out again."
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".

Trump told Russia that Ukraine would not be granted Nato membership and should not expect to regain the territory it has lost
The United States' foreign policy is now driven, in part at least, by the imperatives of its culture wars. The security of Europe has become entangled in the battle between two polarised and mutually antagonistic visions of what the United States stands for.
Some think the division is about more than Trump's particular views and that Europe can not just sit tight waiting for his term in office to end.
"The US is becoming divorced from European values," argues Ed Arnold, senior research fellow at the Royal United Services Institute (RUSI) in London. "That's difficult [for Europeans] to swallow because it means that it's structural, cultural and potentially long-term. "
"I think the current trajectory of the US will outlast Trump, as a person. I think Trumpism will outlast his presidency."
Nato Article 5 'is on life support'
The Trump White House has said it will no longer be the primary guarantor of European security, and that European nations should be responsible for their own defence and pay for it.
"If [Nato countries] don't pay, I'm not going to defend them. No, I'm not going to defend them," the president said earlier this month.
For almost 80 years, the cornerstone of European security has been embedded in Article 5 of the North Atlantic Treaty, which states that an attack on one member state of the alliance is an attack on all.
In Downing Street last month, just before his visit to the White House, the Prime Minister Sir Keir Starmer told me during an interview that he was satisfied that the United States remained the leading member of Nato and that Trump personally remained committed to Article 5.
Others are less sure.

President Trump said earlier this month that he would not defend Nato countries if they failed to meet their financial commitments. (Pictured with Nato Secretary General Mark Rutte)
Ben Wallace, who was defence secretary in the last Conservative government, told me earlier this month: "I think Article 5 is on life support.
"If Europe, including the United Kingdom, doesn't step up to the plate, invest a lot on defence and take it seriously, it's potentially the end of the Nato that we know and it'll be the end of Article 5.
"Right now, I wouldn't bet my house that Article 5 would be able to be triggered in the event of a Russian attack… I certainly wouldn't take for granted that the United States would ride to the rescue."
According to polling by the French company Institut Elabe, nearly three quarters of French people now think that the United States is not an ally of France. A majority in Britain and a very large majority in Denmark, both historically pro-American countries, now have unfavourable views of the United States as well.
"The damage Trump has done to Nato is probably irreparable," argues Robert Kagan, a conservative commentator, author and senior fellow at the Brookings Institute in Washington DC who has been a long time critic of Trump.
"The alliance relied on an American guarantee that is no longer reliable, to say the least".
And yet Trump is by no means the first US president to tell Europe to get its defence spending in order. In 2016 Barack Obama urged Nato allies to increase theirs, saying: "Europe has sometimes been complacent about its own defence."
Has a 'fragmentation of the West' begun?
All of this is great news for Putin. "The entire system of Euro-Atlantic security is crumbling before our eyes," he said last year. "Europe is being marginalised in global economic development, plunged into the chaos of challenges such as migration, and losing international agency and cultural identity."
In early March, three days after Volodymyr Zelensky's disastrous meeting with Trump and Vance in the White House, a Kremlin spokesman declared "the fragmentation of the West has begun".

Zelensky's tense meeting with Trump and Vance at the White House earlier this month prompted a Kremlin spokesman to declare that "the fragmentation of the West has begun"
"Look at Russia's objectives in Europe," says Armida van Rij, head of the Europe programme at Chatham House. "Its objectives are to destabilise Europe. It is to weaken Nato, and get the Americans to withdraw their troops from here.
"And at the moment you could go 'tick, tick and almost tick'. Because it is destabilising Europe. It is weakening Nato. It hasn't gone as far as to get the US to withdraw troops from Europe, but in a few months time, who knows where we'll be?"
'We forgot the lessons of our history'
One of the great challenges Europe, in particular, faces from here is the question of how to arm itself adequately. Eighty years of reliance on the might of the United States has left many European democracies exposed.
Britain, for example, has cut military spending by nearly 70% since the height of the Cold War. (At the end of the Cold War, in the early 1990s, Europe allowed itself a peace dividend and began a decades-long process of reducing defence spending.)
"We had a big budget [during the Cold War] and we took a peace dividend," says Wallace. "Now, you could argue that that was warranted.
"The problem is we went from a peace dividend to corporate raiding. [Defence] just became the go-to department to take money from. And that is where we just forgot the lessons of our history."
The prime minister told parliament last month that Britain would increase defence spending from 2.3% of GDP to 2.5% by 2027. But is that enough?
"It isn't enough just to stand still," argues Wallace. "It wouldn't be enough to fix the things we need to make ourselves more deployable, and to plug the gaps if the Americans left."

Uncertainty remains over whether the US, under Trump, can still be relied upon to defend its Nato allies
Then there is the wider question of military recruitment. "The West is in freefall in its military recruiting, it's not just Britain," argues Wallace.
"At the moment, young people aren't joining the military. And that's a problem."
But Germany's new Chancellor-in-waiting, Friedrich Merz, has said Europe must make itself independent of the United States. And "Europeanising" NATO will require the build up of an indigenous European military-industrial complex capable of delivering capabilities that currently only the United States has.
Others share the view that Europe must become more self reliant militarily - but some are concerned that not all of Europe is on board with this.
"Where we are at the moment is that the East Europeans by and large, don't need to get the memo," says Ian Bond, deputy director, Centre for European Reform. "The further west you go, the more problematic it becomes until you get to Spain and Italy."
Mr Arnold agrees: "The view in Europe now is this isn't really a debate anymore, it's a debate of how we do it and maybe how quickly we do it, but we need to do this now."
Piecing together a new world order
There is a short list of "very important things" that only the United States currently provides, according to historian Timothy Garton Ash.
"These are the so-called strategic enablers," he says. "The satellites, the intelligence, the Patriot air defence batteries, which are the only ones that can take down Russian ballistic missiles. And within three to five years we [countries other than the US] should aim to have our own version of these.
"And in this process of transition, from the American-led Nato [the idea is] you will have a Nato that is so Europeanised that its forces, together with national forces and EU capacities, are capable of defending Europe - even if an American president says 'leave us out of this'."
The question is how to achieve this.
Ms van Rij stresses that, in her view, Europe does need to build a Europe-owned European defence industrial base - but she foresees difficulties.
"What's really difficult are the divisions within Europe on how to actually do this and whether to actually do this."

Former U.S President Harry S. Truman (1884–1972) signs the North Atlantic Treaty, formally establishing NATO
The European Commission and experts have been trying to figure out how this defence may work for several decades. "It has traditionally been very difficult because of vested national interests... So this is not going to be easy."
In the meantime, Trump appears ready to turn the page on the post-Cold War rules-based international order of sovereign states that are free to choose their own destinies and alliances.
What he seems to share with Vladimir Putin is a desire for a world in which the major powers, unconstrained by internationally agreed laws, are free to impose their will on smaller, weaker nations, as Russia has traditionally done in both its Tsarist and Soviet Empires. That would mean a return to the "spheres of interest" system that prevailed for 40 years after the Second World War.
We don't know exactly what Donald Trump would do were a Nato country to be attacked. But the point is that the guarantee of US help can no longer be taken for granted. That means Europe has to react. Its challenge appears to be to stay united, finally make good on funding its own defence, and avoid being drawn into the "sphere of influence" of any of the big powers.
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