While sitting beside Prime Minister Keir Starmer of the United Kingdom in the Oval Office on Thursday, President Trump said repeatedly that he trusted Vladimir V. Putin of Russia not to violate the terms of whatever peace deal that might soon be reached to end the war in Ukraine.
“I think he’ll keep his word,” Mr. Trump said of Mr. Putin. “I’ve known him for a long time now.”
The president referenced the investigations into Russian meddling in the 2016 election that occurred during his first term in office and implied that it all just brought the two leaders closer together. “We had to go through the Russian hoax together,” said Mr. Trump. “They had to put up with that, too. They put up with a lot.”
His attitude toward the Russian leader could hardly be more different from the British leader sitting inches away in the Oval Office.
Mr. Starmer is the latest in a series of European leaders to come to Washington hoping to reason with Mr. Trump as he pushes for negotiations with the Russians to end the war they started in Ukraine. Emmanuel Macron, the French president, visited earlier in the week.
Mr. Starmer had come to the capital with a promise and a plea: He planned to tell Mr. Trump that his country is willing to send troops to Ukraine as part of a peacekeeping effort once the war ends.
But in the Oval Office on Thursday afternoon, Mr. Trump was asked about this hypothetical scenario: What if Britain sent those peacekeeping troops to Ukraine, only for Russia to renege on a peace deal, going on the attack again? Would the Americans come to the aid of the British in Ukraine?
In the span of one minute, Mr. Trump seemed to say no (“They can take care of themselves very well”); and then yes (“If they need help, I’ll always be with the British”); before landing back on no (“They don’t need help”).
“Could you take on Russia by yourselves?” Mr. Trump asked Mr. Starmer, and uneasy laughter broke out in the room. (All this was said while Vice President Vance and Secretary of State Marco Rubio sat on the other side of Mr. Trump.)
Mr. Starmer intended to use his visit to urge Mr. Trump not to abandon Ukraine to the demands of Mr. Putin.
Ukraine has felt the consequences of weak security agreements before: In December 1994 they gave up their Soviet nuclear weapons, which were still controlled from Moscow, and the United States, Britain and Russia agreed to respect the existing borders of Ukraine.
The agreement proved worthless when Russia seized Crimea in 2014. And while the agreement brought the Ukrainians arms and intelligence support in 2022 at the start of Russia’s full-scale invasion, neither Britain nor the United States provided troops.
In the Oval Office, Mr. Starmer thanked Mr. Trump for changing the conversation on Ukraine and said he believed a “historic deal” could be achieved to end the fighting. The prime minister also gave Mr. Trump a letter from King Charles III inviting him to a state dinner.
After opening it, Mr. Trump called the king “a great, great gentleman.”
But underneath the diplomatic pleasantries, the two men were expected to have some difficult conversations about the future of Ukraine and whether a peace agreement results in concessions to Russia despite its invasion of its neighbor three years ago.
Mr. Starmer was prepared to urge Mr. Trump not to rush into a diplomatic resolution to the conflict without ensuring that Ukraine is given security guarantees that would prevent Russia from invading again in the future.
But White House officials said Thursday morning ahead of the meeting that discussions about a deal regarding Ukraine’s rare-earth minerals — which Mr. Trump has said the United States needs — do not include such guarantees.
A White House national security official said the economic partnership with the Ukrainians does not include any specific guarantee of funding for future war fighting. The official, speaking on condition of anonymity to preview the meeting, said the minerals agreement would not commit any American personnel to the region.
That difference was expected to be at the center of the private deliberations between Mr. Starmer and Mr. Trump.