For 70 years, the Nato alliance has kept the peace in Europe but after four years of estrangement with Washington, another Trump administration could have killed it off for good, experts say.
It is accepted that a Joe Biden presidency will save the alliance, bringing more weight against the great power rivals of Russia and China.
The probably renewed emphasis on the transatlantic relationship has led many analysts to welcome the new president.
“I was of the view that four more years of the Trump presidency might kill Nato as an effective alliance because of the strain that he was putting on Nato’s relationships, at a time when the Europeans are struggling anyway,” said Prof Michael Clarke, of the Royal United Services Institute.
Another leading analyst said Mr Trump simply regarded all alliances, unless they substantially benefitted the US, as a swindle.
Dr Dana Allin, a well-connected American academic at the International Institute for Strategic Studies, said Mr Trump requested during an Oval Office discussion to withdraw from Nato.
“There was a real chance that in a second term he would have done that," Dr Allin said.
"Trump had zero interest or commitment to the Nato alliance and, in fact, to the concept of allies.
"He failed to understand the idea that alliances were a source of American strength and saw them as a way for other countries to rip off the United States.”
Dr Allin said damage to other European institutions would be far greater during a second Trump term.
“He was hostile to the European Union as a concept, and obviously very pro-Brexit," he said.
"Now, of course, Brexit has happened but I think the Biden administration is also going to have a very different attitude towards the European Union and towards a relationship with Europe that covers a lot of dimensions, other than military and security, especially on Iran.”
Prof Clarke said Mr Biden would align himself with the three major European powers – France, Germany and Britain – as part of a new nuclear deal with Iran to replace the Joint Comprehensive Plan of Action agreement.
“They won't want the old deal back but he wants a deal on the basis of the JCPOA.”
Nathalie Tocci, special adviser to Josep Borrell, the EU's foreign policy chief, said Mr Trump was the first US president to question Nato’s defence treaty and treat the EU as an adversary in trade.
“Under a Biden administration, much of that nightmare is gone,” Ms Tocci wrote for the Institute for International Affairs.
“The US commitment to Nato will be reaffirmed and transatlantic discussions over European defence will veer away from the mercantilist obsession with trade imbalances, and towards a healthier US concern about European risk and responsibility sharing and the resilience of Nato.”
She said a Biden administration would seek “genuine partnership” across the Atlantic.
“It will welcome European facilitation to ease its way back into the Iran nuclear deal,” Ms Tocci said.
Dr Alan Mendoza, founder of the Henry Jackson Society think tank, said Mr Biden would want to continue Mr Trump’s policy of ensuring the Europeans paid their way in Nato.
“There is no way the Americans are going to come back and say, 'You’re going to get this for free', so the future of Nato is very much in the hands of Europeans,” Dr Mendoza said.
There are warnings that Britain’s plans to introduce the Internal Markets Bill, which could bring in a hard border in Northern Ireland, could also lead to a breach with Washington.
Dr Mendoza said that if Mr Biden had strong concerns about introducing a border and that if the situation were compromised, he would "stand on the side of Ireland, not on the side of the UK".
Dr Allin said that the US would ultimately favour Europe over Britain in any trade deal because the EU was by far a bigger trading partner.
“The United States can never compensate for the loss or diminishment of economic relations with the European Union,” he said.
But with Britain hosting the Cop26 Climate Change conference next year, it is likely that Prime Minister Boris Johnson would be in complete alignment with the Democrat administration on international environment policy.
Dr Mendoz said that the message would be: “Let's work together on that end remind them that Britain is a good partner."
Dr Allin agreed that the US and Europe would be united on climate change.
“It is the single biggest global challenge that the United States and most of Europe now again agree on, which is a very important step,” he said.
Prof Clarke said the new American president would be robust with Europe and the other great powers.
“I think Mr Biden has licence to be a tough-minded president in foreign policy so I think he'll be tough on China and remain tough on China," he said.
"He will be quite tough with Europeans too. He will inherit a certain amount of the Trump agenda but will pursue it in a rather more civilised manner.”
Dr Mendoza said the first meeting between Mr Johnson and Mr Biden would show if there were chemistry between the two.
He said it would be difficult for the UK leader, a Trump supporter, but he and the president-elect were both “decent people” who could well form a strong bond when they got to know each other.
Dr Mendoza said the relationship between Tony Blair and George W Bush was extremely solid despite them coming from different ends of the political spectrum.
“I think both Joe Biden and Boris Johnson are very genial people who when they get in a room together will have the opportunity to forge a new relationship,” he said.