LONDON — President Trump sat down in a gilded chair beside President Emmanuel Macron of France on Tuesday, prepared for what has become a ritual of sorts on his home turf at the White House: He holds forth as another leader is left to smile stoically through his jokes, jabs and insults.
But Mr. Macron changed the script. By the time their 45-minute appearance at the American ambassador’s residence in London was over, the French leader had managed a rare role reversal, putting Mr. Trump on the defensive about his vision for NATO and his handling of a military conflict involving Turkey, and swatting away the president’s joke about sending Islamic State fighters from Syria to France.
“Would you like some nice ISIS fighters?” Mr. Trump said, crouching forward and claiming that “many” fighters had come from France. “I can give them to you.”
“Let’s be serious,” Mr. Macron, who sat coiled on the edge of his seat with one hand clamped firmly on his knee, replied. “The very large numbers of fighters on the ground are the fighters coming from Syria, from Iraq.”
The dramatic moment, which came as both leaders were in London to celebrate the 70th anniversary of the NATO alliance, underscored how a relationship formerly known for lingering hugs, lint-brushing and white-knuckle handshakes has devolved over divisions on matters ranging from terrorism to trade policy.
This time, the deterioration took place on live television.
“The president doesn’t like confrontation in person and doesn’t quite know how to react to being on the receiving end,” said Heather A. Conley, director of the Europe program at the Center for Strategic and International Studies. “Macron understands that. He’s decided the best defense is offense.”
Throughout the day, Mr. Trump made news on a number of fronts in his scattershot way, speaking to reporters for over two hours in total. He castigated Democrats as “unpatriotic” for supporting a possible impeachment, commented on Prince Andrew’s relationship with the disgraced financier Jeffrey A. Epstein — “tough story,” Mr. Trump said of the prince, whom he claimed not to know — and mused that he might punt on a trade deal with China until after the 2020 election.
But it was Mr. Macron who was his focus for much of the day.
In November, Mr. Macron — another leader who enjoys talking — made headlines for lamenting, in an interview with The Economist magazine, what he said was the loss of American leadership, leading to “the brain death of NATO.” He said the United States under Mr. Trump appeared to be “turning its back on us,” notably by pulling troops out of northeastern Syria without notice, and called on Europeans to do more in their own defense with the aim of “strategic autonomy.’’
“You have no coordination whatsoever of strategic decision-making between the United States and its NATO allies,” Mr. Macron said. “None.”
His remarks at the time drew a host of rebukes from other NATO allies, including from Chancellor Angela Merkel of Germany, who called Mr. Macron’s “sweeping judgments” unnecessary. Experts said the French leader had made a valid point, but warned that his words would be weaponized by Mr. Trump, who on Tuesday jumped at the opportunity.
In a 52-minute meeting Tuesday morning with Jens Stoltenberg, the NATO Secretary General, Mr. Trump called Mr. Macron’s comments last month “very insulting” and a “very, very nasty statement essentially to 28 countries.”
But when asked during the afternoon meeting to address his earlier comments about Mr. Macron, Mr. Trump, a leader averse to face-to-face confrontation, initially demurred. When it was his turn to speak, Mr. Macron was direct.
“My statement created some reactions,” Mr. Macron said. “I do stand by it.”
As they continued a terse back-and-forth, Mr. Macron targeted Mr. Trump’s relationship with President Recep Tayyip Erdogan of Turkey. Mr. Erdogan has already upset NATO allies by purchasing a sophisticated Russian antiaircraft missile system, the S-400. He is now threatening to oppose NATO’s plans to fortify the defense of Poland and the Baltic countries if the alliance does not join him in labeling some Kurdish groups as terrorists.
Mr. Trump has maintained a soft touch with Mr. Erdogan, including giving the green light for the Turkish leader to advance troops into Northern Syria in an offensive against Kurdish-led forces that had been allied with the United States. On Tuesday, Mr. Trump would not say whether he would impose sanctions on Turkey for buying the missile system.
Mr. Macron was harsher, saying that the purchase would need to be explained and that there would need to be common ground on classifying different groups of fighters as terrorists.
“When I look at Turkey, they are fighting against those who fight with us,” he said. “Who is the enemy today?”
During several appearances before reporters, Mr. Trump found several chances to effectively highlight a foreign policy achievement he hopes will help his re-election campaign: getting allies to pay more toward the costs of running NATO.
“What I’m liking about NATO is that a lot of countries have stepped up, really, I think, at my behest,” Mr. Trump said.
But Mr. Macron made it clear that while he wanted more military spending by European countries, NATO had other challenges to address besides “just numbers.”
“I’m sorry to say that we don’t have the same definition of terrorism around the table,” Mr. Macron said. Throughout the icy exchange, clear disagreements between the two leaders over how to deal with Mr. Erdogan’s approach in Syria even overshadowed the issue of France imposing taxes on American tech giants, and a potential retaliatory tariff on French wines.
“We tax wine and we have other taxes scheduled,” Mr. Trump said. “But we’d rather not do that. But that’s the way it works.”
Mr. Trump slumped back in his chair, while Mr. Macron sat on the edge of his chair, bobbing and gesturing energetically.
Mr. Macron’s aggressive approach appeared at times to unsettle Mr. Trump, who said at one point that he did not support Iranian protesters who are calling for an end to the Islamic Republic’s government and the downfall of its leaders. That appeared to contradict his own administration’s position and the recent comments of Secretary of State Mike Pompeo, but Mr. Trump quickly clarified his position with a tweet saying that the “United States of America supports the brave people of Iran.”
In the past, Mr. Trump has been so disruptive at NATO meetings that he triggered an emergency session. He has accused other member countries of shortchanging the United States on military spending, and he has called the alliance “obsolete,” questioning whether it still served a purpose.
A goal of the current meeting was to avoid any formal disruptions. But Ian Lesser, a former American official who directs the Brussels office of the German Marshall Fund, said that the subtext of Mr. Macron’s criticism of NATO was growing doubt about Washington’s commitment.
“When he says ‘brain death,’ Macron’s talking about American leadership,’’ Mr. Lesser said. “That’s perhaps what Trump is hearing. Trump’s view is that he’s turned NATO around, and the rhetoric from Paris is compromising that narrative.’’
Experts in the region said that with Britain moving toward leaving the European Union and the German government enmeshed in its own political troubles, Mr. Macron saw an opportunity to assert French leadership in Europe.
“President Macron is seizing that moment, seeking to be disruptive in his own way, and so we will see how that works,” Ms. Conley said. “He’s increasingly isolating himself within Europe.”
In the background of these competing global interests is Mr. Trump’s possible impeachment. On Wednesday, the House Judiciary Committee is set to question legal experts about whether there are grounds to impeach Mr. Trump for pressuring Ukraine to take actions that could help him in the 2020 election.
Several times through the day, Mr. Trump turned his attention back to impeachment, castigating the effort led by Democrats as “unpatriotic. ”And he again defended his behavior during a July call with the Ukrainian President, Volodymyr Zelensky — an interaction that formed the basis for the inquiry.
“I did nothing wrong,” Mr. Trump said of the impeachment inquiry during a bilateral meeting with Mr. Stoltenberg, adding that he did not feel that a censure from Congress was justified, either. “You don’t censure somebody when they did nothing wrong.”
On Tuesday, Mr. Trump also met with Prime Minister Justin Trudeau of Canada and hosted a private fund-raising round table with supporters, which Trump campaign officials said raised $3 million.
Notably absent from the president’s schedule was a one-on-one meeting with the British prime minister, Boris Johnson, who is campaigning ahead of a Dec. 12 election and has been desperate to keep Mr. Trump at arm’s length.
Mr. Johnson is managing the political fallout from a terrorist attack on Friday in central London, where a lone extremist fatally stabbed two people and wounded three others.
A chief concern in Britain is that Mr. Trump could change the course of next week’s election, intentionally or not, by sending inflammatory tweets or wading into local politics in interviews. On Tuesday, Mr. Trump indicated that he would respect Mr. Johnson’s wishes and not interfere in the impending election.
“I’ll stay out of the election,” Mr. Trump said. “I think Boris is very capable and he will do a good job.”
Steven Erlanger contributed reporting.