LONDON — President Trump put his brand of confrontational and disruptive diplomacy on full display on Thursday, unsettling NATO allies with a blustering performance in Brussels and then, in a remarkable breach of traditional protocol, publicly undercutting Prime Minister Theresa May of Britain hours after landing in her country.
NATO, a pillar of the global order, emerged from its two-day confrontation with Mr. Trump on Thursday intact but distracted and rattled, a further challenge to the alliance as it faces an expansionist Russia and growing authoritarianism among some of its own members.
Wrapping up talks with fellow leaders of the 29-nation trans-Atlantic alliance, Mr. Trump reaffirmed support for NATO, but only after stirring more discord with a vague threat that the United States could go its own way if the allies resisted his demands for additional military spending, capping a summit meeting punctuated by his escalating complaints.
He then flew to Britain, where the newspaper The Sun published an interview that in its bluntness and criticism of Mrs. May on her own turf all but dispensed with diplomatic norms.
In the midst of a week that has seen Mrs. May parrying threats to her hold on power, Mr. Trump second-guessed her handling of the main issue on her plate: how Britain should cut its ties to the European Union. He cast doubt on whether he was willing to negotiate a new trade deal between Britain and the United States and praised Mrs. May’s Conservative party rival, Boris Johnson, as a potentially great prime minister.
The interview was published as Mr. Trump and Mrs. May were wrapping up what appeared to be a chummy dinner at Blenheim Palace — earlier, they had walked inside holding hands — and a day ahead of the president’s scheduled meeting with Queen Elizabeth II at Windsor Castle. There was no immediate response from the British government.
“Well I think the deal that she is striking is not what the people voted on,” Mr. Trump said in the interview, speaking of the approach Mrs. May is taking to Britain’s exit from the European Union, or Brexit, under which the British economy would effectively continue to be subject to many European regulations.
Speaking of Mr. Johnson, who resigned this week as foreign secretary in protest over Mrs. May’s Brexit strategy and who has long been seen as likely to challenge her for her job, Mr. Trump said, “Well I am not pitting one against the other. I’m just saying I think he would be a great prime minister. I think he’s got what it takes and I think he’s got the right attitude to be a great prime minister.”
The day amounted to a global disruption tour unlike anything undertaken by any other recent American leader, and it seemed to be a new twist on the “special relationship” between Britain and the United States.
Mixing pique, self-congratulation and a relentless focus on whether the United States is being taken advantage of by its closest allies, Mr. Trump spent his final hours at the NATO meeting in Brussels bludgeoning other leaders but got little in the way of concrete results.
In the latest example of his penchant for creating conflict to draw attention to his agenda, he first demanded an emergency meeting to address his grievances and then called a news conference — something he has not done in the United States in more than a year — to claim “total credit” for having pressed NATO members into increasing their military budgets “like they never have before.”
It was a classic Trump performance — bluster, confrontation and demands followed by a unilateral declaration of victory — but his claim was quickly dismissed by the leaders of Italy and France, who disputed that they had made any new pledges for increasing spending, adding to the sense of disarray.
His willingness to criticize Mrs. May in her own country only underscored the tensions in his relations with other allied leaders and his totally unconventional approach to diplomacy.
But the NATO summit meeting did produce some substantive accomplishments for those who support the alliance’s traditional focus on maintaining security against Russia. The leaders, including Mr. Trump, had signed on to a statement — issued after the first day of the meeting rather than at its conclusion, reducing the chances that the president might change his mind about it — that highlighted agreement on a plan to improve the readiness and mobility of the armed forces across the Continent and progress on issues like cybersecurity. And they agreed on tough language aimed at Moscow, especially regarding Russia’s annexation of Crimea.
But with Mr. Trump scheduled to meet with President Vladimir V. Putin of Russia on Monday — and with the Trump campaign’s ties to Russia the focus of a wide-ranging investigation — the gathering of the leaders was dominated not by discussion over how to address security threats or Mr. Putin’s efforts to divide the West but by wrangling over money.
Rather than projecting unity ahead of the Trump-Putin meeting, the gathering generated nonstop images of division. Intense concern about the corrosive effects of populism and growing authoritarianism in NATO members like Turkey, Hungary and Poland on support for post-World War II institutions and policies received little or no sustained public attention.
In the weeks before the summit meeting, alliance leaders feared that Mr. Trump would try to blow everything up, dealing a truly severe blow to the multilateral world order and to trans-Atlantic deterrence and cohesion.
On Thursday, Mr. Trump dismissed concerns that his relationship with Russia was too cozy, or that his hardball tactics at NATO had played into the hands of Mr. Putin, whom he is to meet in Helsinki, Finland, next week. But after 48 hours of overt conflict with allies — and the second international summit meeting in two months where he has sparred openly with European leaders — he said he looked forward to a positive encounter with the Russian president.
“I hope that we’re going to be able to get along with Russia; I think that we probably will be able to,” Mr. Trump said. “We go into that meeting not looking for so much.”
The White House hastily called the news conference in Brussels amid reports that Mr. Trump had unleashed a tirade during a closed-door morning meeting against member countries he complained were still not spending enough on their militaries. Mr. Trump used the news conference to hail himself, again, as a “stable genius,” saying he deserved “total credit” for pushing the allies to increase their military spending by more than previously agreed to.
According to a person briefed on the meeting, Mr. Trump told other NATO leaders that if their countries did not meet the 2 percent standard by January, the United States would go it alone, a comment that some interpreted as a threat to withdraw from the alliance. (
White House officials ignored requests to clarify what the president had said or offer any information on which the allies had responded to his threats with promises of more military spending.
In the end, though, he spoke supportively of the alliance, saying: “I believe in NATO. I think NATO’s a very important — probably the greatest ever done.”
President Emmanuel Macron of France said, “We are all leaving this summit stronger because the president of the United States of America reaffirmed his commitment and his desire to have a strong NATO.”
But the leaders were clearly taken aback by Mr. Trump’s disruptive posture. They had fair warning from his confrontational behavior last month at the Group of 7 summit meeting in Canada, where he called the host, Prime Minister Justin Trudeau, “Very dishonest and weak” in a Twitter post.
Nevertheless, they emphasized that in general Mr. Trump was conciliatory in private sessions, though he was clearly upset by Thursday headlines indicating that he was happy with the NATO discussion on spending. To express his displeasure, he came late to the scheduled meetings and insisted on the emergency session on spending.
Mr. Trump wanted to underline that he was not at all happy with the level of burden-sharing despite having agreed to the communiqué, and he urged countries lagging behind to move more quickly, according to a senior official who attended the special session. Alliance members have pledged to raise military spending to 2 percent of G.D.P. by 2024.
The discussion was a healthy one, the official said. It created more urgency and it also allowed the summit meeting to end with an agreed message on improving spending, far better than an outcome where Mr. Trump was vocally unhappy and the others were perceived not to care, he added.
Indeed, Mr. Trump came away mollified, broadcasting his own sense of triumph, though he had said the day before that member nations had to reach the 2 percent goal immediately, and that the target should rise to 4 percent.
“In the end I think the meeting was less divisive than feared,” said Alexander Vershbow, a former NATO deputy secretary general. “I think it’s the reality show that the president loves. There wasn’t enough drama, so Trump has a tantrum, knocks over the table, and allies are used as props in his reality show.”
That being said, he added, “NATO goes on.”
The substance of the meeting, he and others said, is in the communiqué. That document, a product of nearly a year of work, commits the alliance to a stronger deterrent against Russia, more efforts on cybersecurity, a strengthening of the alliance’s southern strategy and a new training program for Iraq, Tunisia and Libya. It also called for nations to devote at least 20 percent of their growing military budgets to equipment and modernization.
The United States pays 22 percent of NATO’s budget, which covers things like offices, salaries and some equipment used in joint operations. According to the International Institute for Strategic Studies, of the $603 billion that the United States spends on the military each year, about $31 billion goes to Europe.
In the end, some analysts criticized Mr. Trump for fabricating a ruckus that, however much it might serve his political ends, was harmful to the alliance.
“NATO always had a good story to tell at this summit, with the communiqué reflecting a robust and resilient alliance that is making real progress on a range of challenges,” said Amanda Sloat, a former State Department official now at the Brookings Institution. “Trump’s belligerent tweets and taunts unfortunately overshadowed what should have been a straightforward message of success.”
The drama in Brussels on Thursday was all about Mr. Trump’s desire to make noise for his political base, Ms. Sloat said. “Some say there was some victory for Trump, that he achieved what he wanted, but it’s not true,” she said. “There is no utility in creating all this noise, it’s incredibly divisive.”
It is good to push for more defense spending, but allies are not like business partners, she said. “You can’t lambaste Germany one day and then ask them for something else the next,” she said. “Actions and rhetoric have consequences. Blowing up summits and projecting disunity doesn’t help credibility.”
Then Mr. Trump moved on to Britain.
Steven Erlanger and Julie Hirschfield Davis reported from Brussels, and Katie Rogers from London. Aurelien Breeden contributed reporting from Paris, Gaia Pianigiani from Rome and Stephen Castle from London.