BRUSSELS — President Trump upended a summit here to admonish leaders and demand they quickly increase their defense spending, although he ultimately reaffirmed U.S. support for NATO on Thursday.
Trump’s ambush jolted the transatlantic alliance, and some diplomats perceived his comments as threatening a U.S. withdrawal from NATO. But Trump later declared in a news conference, “I believe in NATO,” and, as he prepared to depart Brussels, he reiterated that the United States is committed to its Western allies.
“I told people that I’d be very unhappy if they did not up their commitments very substantially,” Trump told reporters after the meeting. “Everyone’s agreed to substantially up their commitment. They are going to up it at levels never thought of before.”
NATO member nations committed in 2014 to each spend 2 percent of gross domestic product on defense by 2024, though only eight countries have been on track to reach that goal this year. It was not immediately clear what specific new commitments were made here.
• Mr. Trump said on Thursday that NATO allies had agreed to his demand for a significant increase in military spending. But he offered no specifics, and leaders of some other NATO nations rejected the claim.
“Ultimately, that will be going up quite a bit higher than that,” Trump said at the news conference, after privately calling Wednesday on leaders to double their commitments to 4 percent of gross domestic product.
Mr. Trump strongly recommitted American support for NATO, a bedrock of Western security policy for generations, on Thursday, comments that at least temporarily calmed fears that he might move toward dismantling the alliance.
“The United States commitment to NATO is very strong, remains very strong,” he said at a news conference in Brussels. “I believe in NATO.”
According to a person briefed on Mr. Trump’s meeting with other NATO leaders, Mr. Trump said that if the other countries did not increase military spending to 2 percent of their economic output by January, the United States “would go it alone.”
Mr. Trump has not held a news conference on American soil for more than a year. But on Thursday, flanked by Secretary of State Mike Pompeo and John R. Bolton, his national security adviser, he took questions from reporters for over a half an hour.
He dismissed any concern that his relationship with Russia was too cozy or that his relationship with allies was too harsh.
Mr. Trump’s first summit meeting with the Russian president will be parsed for countless layers of meaning.
The West’s stance toward Russia is, as always, a central topic at the NATO meeting, and the United States’ European allies are worried that Mr. Trump aims to reduce the American security role in dealing with Moscow.
Russia is waging a proxy war against Ukraine, has forcibly annexed part of that country, has meddled in other nations’ elections, gives crucial support to the Syrian president, Bashar al-Assad, and stands accused of using a chemical weapon on British soil.
Mr. Trump said that other NATO countries had agreed to significant increases in military spending in response to his demands.
But within a few hours, Emmanuel Macron, the French president, and Giuseppe Conte, Italy’s prime minister, said the allies had simply agreed to keep a 2014 commitment to increase military spending to 2 percent of gross domestic product by 2024.
“A communiqué was issued yesterday,” Mr. Macron told reporters after the meeting in Brussels. “This communiqué is clear. It reaffirms the 2 percent by 2024 commitments. That’s all.”
Mr. Conte said: “Italy inherited spending commitments to NATO, commitments that we did not change, so no increase in spending. As far as we’re concerned, today we did not decide to offer extra contributions with respect to what was decided some time ago.”
Angela Merkel, the German chancellor, said that her country would consider more spending, but said nothing about making any new commitments. And she undercut the notion that reconsideration of Germany’s defense budget was due simply to American pressure.
“Given the discussion of many European allies here, not just the American discussion, I think we have to ask again and again what else can we possibly do,” she said.
Mr. Trump has insisted that NATO countries meet the 2 percent threshold right away, and that the long-term target be doubled to 4 percent.