LONDON — A once-cordial relationship between President Trump and President Emmanuel Macron of France devolved in a dramatic fashion on Tuesday, as the two leaders publicly sparred over their approach to containing the threat of terrorism and a shared vision for the future of NATO, a 70-year-old alliance facing existential threats on multiple fronts.
In a lengthy appearance before reporters, the president met a cool reception from Mr. Macron, who earlier in the day Mr. Trump derided as “very insulting” for his recent remarks on the “brain death” of the alliance. When asked to address his earlier comments on the French leader, Mr. Trump, a leader averse to face-to-face confrontation, initially demurred, but Mr. Macron was direct.
“My statement created some reactions,” Mr. Macron said. “I do stand by it.”
What followed was an extended, terse back-and-forth over trade, immigration, and Mr. Trump’s relationship with President Recep Tayyip Erdogan of Turkey.
Mr. Trump’s interactions with the Turkish president are also sure to be closely watched. Mr. Erdogan, who has already upset NATO allies by purchasing a sophisticated Russian antiaircraft missile system, the S-400, is now threatening to oppose NATO’s plans to update the defense of Poland and the Baltic countries if the alliance does not join him in labeling some Kurdish groups as terrorists.
”Who is the enemy today?” Mr. Macron asked. “And let’s be clear and work together on that.”
The meeting continued to devolve as the two discussed the containment of ISIS fighters in Syria. Hunched forward, Mr. Trump tried to jokingly offer captive fighters to the French.
“Would you like some nice ISIS fighters?” Mr. Trump said.
“Let’s be serious,” a stone-faced Mr. Macron replied. Mr. Macron said that he and Mr. Trump “don’t have the same definition of terrorism around the table.”
“When I look at Turkey, they are fighting against those who fight with us,” he added, referring to Kurdish fighters.
The contentious tone was baked into the day’s proceedings. Hours earlier, in a meeting with Jens Stoltenberg, the secretary general of NATO, Mr. Trump said President Emmanuel Macron of France had been “very insulting” to the alliance.
Mr. Macron had suggested that Europe could no longer assume unwavering support from the United States. “I think nobody needs it more than France,” Mr. Trump said of the alliance, “and 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.”
Mr. Trump’s visit comes as leaders across Europe struggle to balance the shared goal of combating the rising influence of global adversaries — China will be a focus — and containing other unpredictable members, including President Recep Tayyip Erdogan of Turkey.
On Tuesday, Mr. Trump said that he was considering delaying reaching a deal in his protracted and economically damaging trade war with China until after the 2020 election.
“In some ways I like the idea of waiting until after the election for the China deal,” Mr. Trump said, adding that he had “no deadline” for reaching an accord.
Mr. Trump’s defense of NATO against Mr. Macron’s comments was something of a role reversal for the two leaders. 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 questioned whether the alliance still served a purpose.
A goal of the current meeting was to avoid any formal disruptions. This time, however, it was Mr. Macron’s comments that were viewed as unhelpful to the alliance.
Mr. Trump called the remarks a “very, very nasty statement essentially to 28 countries” and said that NATO served a “great purpose.”
Heather A. Conley, director of the Europe program at the Center for Strategic and International Studies, said Mr. Macron saw an opportunity to assert French leadership in Europe, with Britain moving toward leaving the European Union and the German government enmeshed in its own political troubles.
“President Macron is seizing that moment, seeking to be disruptive in his own way, and so we will see how that works,” she said.
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.
That threatens to throw off Mr. Trump’s focus and overshadow a victorious message that administration officials brought along with them to Britain: Last week, officials told reporters that the president had been “spectacularly successful” in urging allies to increase their military spending by more than $100 billion.
On Tuesday morning, Mr. Trump spoke to reporters for 52 minutes, at times turning his attention back to domestic issues. He castigated the impeachment effort led by Democrats as “unpatriotic” and 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, noting that he was not open to a censure from Congress, either. “You don’t censure somebody when they did nothing wrong.”
Mr. Trump’s morning comments set a tense backdrop for his meeting on Tuesday afternoon with Mr. Macron, who has shifted from a charm offensive with Mr. Trump to a more confrontational approach.
Experts in the region said they were watching to see whether Mr. Macron and Mr. Trump could agree on a path forward for NATO. “We need U.S. leadership in order to push any number of things on the NATO agenda, particularly in tougher areas like nuclear modernization or arms control,” Ms. Conley said.
Mr. Trump will also meet with Prime Minister Justin Trudeau of Canada and host a private fund-raising round table with supporters, which Trump campaign officials say will raise $3 million.
Notably absent from the president’s schedule is 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.
Mr. Johnson will host several leaders, including the president, in a group reception at 10 Downing Street on Tuesday evening, before the Trumps head to Buckingham Palace for a reception with Queen Elizabeth II and Prince Charles.
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.
During his meeting with Mr. Stoltenberg, 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.”
Hours before he and the first lady, Melania Trump, were expected at the palace, Mr. Trump also addressed a controversy engulfing the Royal family. Prince Andrew, the queen’s third child, recently spoke with the BBC about his relationship with the disgraced financier Jeffrey A. Epstein — an interview that turned into a public relations disaster, leading to the prince stepping back from public life.
“I don’t know Prince Andrew, but that’s a tough story,” Mr. Trump said.
In dealing with Mr. Erdogan, Mr. Trump has taken a soft touch, after other NATO members condemned Turkey’s decision to launch an offensive into northeastern Syria against Kurdish militia. A Kurdish force had been fighting alongside the Americans against the Islamic State, but Mr. Trump gave the go-ahead for the Turkish incursion in a controversial phone call.
Steven Erlanger contributed reporting.