LONDON (WASHINGTON POST) - United States President Donald Trump openly jousted with French President Emmanuel Macron - a leader who until recently had been one of Mr Trump's earliest and most prominent partners in bromance.

He thrust Canadian Prime Minister Justin Trudeau uncomfortably into the spotlight, dubbing Mr Trudeau's country "slightly delinquent" and asking for Canada's "number" on meeting its financial commitment to Nato's shared defence.

And he previewed a likely confrontation with German Chancellor Angela Merkel, with whom he is scheduled to meet on Wednesday (Dec 4), over Germany's financial contributions to Nato.

On the first day of the Nato 70th anniversary summit in London, Mr Trump pronounced, prodded and pushed America's allies into a state of unbalance - seizing the global stage to both bully and banter.

To watch Mr Trump perform alongside other world leaders was to witness his use of disequilibrium as political strategy, deployed throughout his presidency to keep everyone slightly off-kilter.

Over the course of three one-on-one meetings with Nato Secretary-General Jens Stoltenberg, Mr Macron and Mr Trudeau, Mr Trump turned what were expected to be brief photo opportunities into his own personal daytime cable show.

As the other leaders largely bore witness, the US President - frequently affable, occasionally bored - held forth for a collective two hours, fielding questions on topics ranging from the impeachment investigation he left at home to the British election campaign he flew into here.

Mr Stoltenberg - praising Mr Trump's "leadership on defence spending" as helping to add more than US$130 billion (S$177 billion) to military coffers from Canada and European allies since 2016 - seemed to acknowledge the ways in which Mr Trump's often capricious, freewheeling style has simultaneously distressed allies while, at times, also yielding results.

"This is unprecedented," Mr Stoltenberg said. "This is making Nato stronger. And it shows that this alliance is adapting, responding when the world is changing."

Mr Trump began the day on a fiery note, using his first public appearance alongside Mr Stoltenberg to criticise Mr Macron for an interview with the Economist magazine, in which Mr Macron described the "brain death" of Nato resulting from the US' decision to not consult with its allies on pulling troops out of Syria.

Mr Trump called the remarks "very, very nasty", "very disrespectful" and "very insulting".

"You just can't go around making statements like that about Nato," said Mr Trump, who has spent much of his time in office deriding the organisation.

"I would say that nobody needs Nato more than France," he said. "That's why I think when France makes a statement like they made about Nato, that's a very dangerous statement for them to make."

Later, during his meeting with Mr Trudeau, he again chided Nato countries that have failed to spend 2 per cent of gross domestic product on the alliance's shared defence, and he twice pressed Mr Trudeau for exactly how much Canada was contributing.

"We'll put Canada on a payment plan," Mr Trump said. "I'm sure the Prime Minister would love that."

He added that the gathered leaders would be discussing what to do with "delinquent" countries, but said he personally prefers to retaliate through trade measures.

"I think it's very unfair when a country doesn't pay, so most likely I'd do something with respect to trade," the US President said.

At his afternoon appearance with Mr Macron, Mr Trump and the French President articulated their disagreements - at times quite forcefully - but with a veneer of conciliation.

Mr Trump said they had a "minor dispute" that he expected they could probably work out - a reference to France's plan to tax US tech giants and the US threat to retaliate with tariffs as high as 100 per cent on some French goods.

"But we would rather not do that," Mr Trump said. "But it's either going to work out or we'll work out some mutually beneficial tax. And the tax will be substantial, and I'm not sure it will come to that, but it might."

Mr Macron reiterated that he was not backing down from his comments to The Economist.

"I know that my statements created some reactions," Mr Macron said. "I do stand by (them)."

He added: "When we speak about Nato, it's not just about money. We have to be respectful with our soldiers. The first burden we share, the first cost we pay, is our soldiers' lives."

Mr Trump then turned to Mr Macron and suggested that Europe - and the French President in particular - should shoulder more responsibility for taking back captured Islamic State in Iraq and Syria (ISIS) fighters.

"Would you like some nice ISIS fighters?" Mr Trump asked with a smirk. "I can give them to you."

The question prompted a scolding from Mr Macron.

"Let's be serious," Mr Macron said. "A very large number of fighters you have on the ground are ISIS fighters coming from Syria, from Iraq and the region. It is true that you have foreign fighters coming from Europe, but this is a tiny minority of the overall problem we have in the region."

The exchange ended with Mr Trump teasing Mr Macron again.

"This is why he's a great politician," the US President said. "Because that is one of the greatest non-answers I've ever heard, and that's OK."

The back-and-forth was unusual in Mr Macron's willingness to challenge Mr Trump, repeatedly, to his face and at times in the middle of his sentences.

At the outset of their relationship, the two leaders were far friendlier - there was even talk of a bromance.

"Macron's willingness to humour Trump tended to paper over some of the disagreements that were bubbling underneath the surface between the two countries," said Dr Amanda Sloat, a former Obama administration official and Europe expert at the Brookings Institution.

But the French leader's approach failed to swing Mr Trump on any policy issues - a difficulty that was thrown under an unforgivingly bright light after Mr Trump pulled out of the Iran nuclear deal last year.

"The overall lesson is that using different means of approaching Trump don't seem to be effective at changing his mind on policy issues," she said, so leaders are increasingly willing to take him on directly.

By nightfall in London, Mr Trump and Mr Macron appeared to have put the day's acrimony aside.

The men arrived together at 10 Downing Street for a reception hosted by British Prime Minister Boris Johnson, with Mr Trump apparently giving Mr Macron, along with Italian Prime Minister Giuseppe Conte, a lift in "The Beast", the President's armoured limousine.

They were serenaded by a Christmas choir as they entered the famous British townhouse together.

In the lead-up to the Nato meeting, anxious policymakers worked to tailor what they could offer up to the US President, searching for ways to demonstrate their willingness to spread around security spending more equitably.

Many leaders still have bitter memories of Mr Trump's hijacking of a session at the last summit, in July 2018, when he broke open a discussion about Ukraine to demand that leaders commit on the spot to upping their defence spending.

This year, several leaders will enter the formal Nato meeting on Wednesday with strategies inside their thick briefing books about what to do if Mr Trump threatens to quit Nato on the spot, according to a senior Nato diplomat who spoke on the condition of anonymity to discuss sensitive preparations.

But a second senior Nato diplomat said Mr Trump's Nato rhetoric on Tuesday seemed more positive than at earlier moments about the fundamental need for the alliance, and added that many policymakers were calmed by the President's relatively good cheer heading into the meeting.

That was welcome news for some of Nato's frontline countries, who have stared down neighbouring Russia since 2014 and have tried to demonstrate to the Kremlin that they had the full backing of the alliance if they were attacked.

"If we are weak, if we are wobbly, then the threat can go up," Estonian Defence Minister Juri Luik said on Tuesday.

During his three impromptu news conferences on Tuesday, Mr Trump tackled a slew of other issues, both foreign and domestic.

Asked about North Korea's continued missile tests, the US President was sanguine, saying the country would "be in a war right now if it weren't for me".

"I have confidence in him," he said, referring to North Korean leader Kim Jong Un. "I like him, he likes me, we have a good relationship."

Still, Mr Trump added: "He definitely likes sending rockets up, doesn't he? That's why I call him Rocket Man."

He also said he did not think his position at Nato was weakened because of the impeachment inquiry directed at his presidency. He dismissed it as a political ploy by Democrats hoping to defeat him in 2020.

Still, asked whether impeachment has cast a cloud as he tries to negotiate with other world leaders, Mr Trump briefly turned pensive.

"Does it cast a cloud? Well, if it does, then the Democrats have done a very great disservice to the country, which they have," he said. "They've wasted a lot of time."

As the afternoon passed, Mr Trump seemed less preoccupied with impeachment and more comfortable in his role as the unofficial emcee of Nato.

Referring to the House Judiciary Committee hearings on impeachment, which begin Wednesday, Mr Trump said he was going to be too busy at the summit to take in the controversy at home.

"I'm not going to watch, but I'm going to be doing this," he said. "It's much more exciting."