A second presidential term for Donald Trump could see the end of the Nato alliance, a group of senior American and French academics say.
The concerns arose after Mr Trump accused Berlin, one of America's most important allies, of failing to meet Nato's spending targets and said the US would cut its troop presence in Germany by more than one quarter.
“So, we're protecting Germany and they're delinquent,” he said on Monday. “That doesn't make sense.
"So I said, 'We're going to bring down the count to 25,000 soldiers'.
“They treat us very badly on trade.”
The planned reduction of 9,500 troops undermines the Nato alliance, which has proved important for post-war European security.
US and French academics and senior retired officials have said another term in office for Mr Trump could fatally weaken an alliance that deters Russian aggression and protects the international rules-based order.
Leading defence academic Michael Clarke was told of the concerns during a conference at the US Centre for Strategic and International Studies last week.
"The Americans were saying that if Trump wins in November, then Nato would be in serious, serious trouble," Mr Clarke, of the Royal United Services Institute, told The National.
"It would be really difficult to hold the whole thing together. Another Trump presidency would so undermine the transatlantic relationship that it would be hard to see what was left of Nato.
"It would be really, really bad for relations because he doesn’t get it, he’s not prepared to see transatlantic relations as previous presidents have.”
The next presidential election is on November 3 and it is not clear if Mr Trump will be able to withdraw the troops by then.
Fellow Republicans in Congress have already argued that a cut would benefit Russia.
The view that Nato could “break” under another Trump term was supported by Bastian Giegerich, of the International Institute for Strategic Studies.
“You can repair the damage from a four-year term but in eight years a lot will break,” Mr Giegerich said.
“If Trump gets re-elected and he continues on this path of treating Nato like a protection racket, I think that undermines the heart of Nato, which is predictable and respectful behaviour.
"Without that trust, Nato will find it hard to exist.”
Mr Trump’s complaint about underspending is accurate. Many of Nato’s 30 member states, including Germany, have not reached the set target of spending 2 per cent of GDP on defence.
The US spent 3.4 per cent last year compared with Germany’s 1.4, although its defence budget is increasing.
Mr Giegerich said polling showed Germans would be in favour of an increase in their defence budget.
“But this should be a conversation not based on threats and on hectoring from Donald Trump," he said.
"The best argument you can make to prevent more defence spending in Germany now is to say you just want to spend money for Donald Trump. He’s actually not helping.”
Berlin has yet to give an official response.