And he held back from the one pledge NATO leaders most wanted to hear: an unconditional embrace of the organization's solemn treaty commitment that an attack on a single alliance nation is an attack on all of them.
Instead, European leaders gazed unsmilingly at Trump while he said that "23 of the 28 member nations are still not paying what they should be paying and what they are supposed to be paying," and that they owe "massive amounts" from past years — a misstatement of NATO's spending targets, which guide individual nations' own domestic spending decisions.
The harsh tone had a toll, as Trump was left largely on his own after the speech as leaders mingled and laughed with each other, leaving the U.S. president to stand silently on a stage ahead of a group photo.
The long day of gruff Brussels meetings was a contrast to his friendlier encounters in the Middle East, where Trump last weekend embraced the authoritarian Saudi monarchy and said he had been wowed by King Salman's wisdom.
In Brussels, Trump sat in a morning meeting with top European Union leaders where one emerged to say that his message to Trump was that the West should concentrate more on values such as human rights and less on "interests." The president lunched with French President Emmanuel Macron, an encounter in which the two leaders shook hands in a tense, white-knuckle embrace. And he sped across Brussels to NATO, where British Prime Minister Theresa May, the leader of Washington's closest ally, buttonholed him over intelligence leaks following Monday's terrorist attack in Manchester, England.
White House press secretary Sean Spicer, traveling with the president, played down the absence of Trump's formal commitment to security guarantees during the speech, saying that there was no question of U.S. support for NATO and all of the obligations that are entailed in membership.
"Having to reaffirm something by the very nature of being here and speaking at a ceremony about it is almost laughable," Spicer said after the speech.
In a news conference after the leaders had held a working dinner and Trump had departed for a meeting of the Group of Seven in Sicily, NATO Secretary General Jens Stoltenberg said, "We have seen that plain speaking of President Trump before."
Even if Trump did not say it, Stoltenberg said, he "has been clear on his commitment to NATO. But President Trump has also been clear in the message to all allies that we have to deliver on the pledge we made to increase defense spending. He was blunt on that message today."
Leaders offered modest applause at the end of a speech that Trump began by asking for a moment of silence in remembrance of the victims of Monday's terrorist attack in Manchester, which killed 22 and wounded many more.
Addressing the British, Trump said, "May all the nations here grieve with you and stand with you." The attack, he said, "demonstrates the depths of the evil we face with terrorism."
Trump did not refer to the British prime minister's irritation, expressed earlier in the day, over what officials in Britain have said was the leak to U.S. news media of intelligence information that Britain gathered in the investigation of the Manchester case and shared with the United States.
"We have strong relations with the United States, our closest partner," May told reporters as she entered NATO's $1.2 billion new headquarters for the ceremony, "and that is, of course, built on trust. Part of that is knowing intelligence can be shared confidently, and I will make clear to President Trump that intelligence shared with law enforcement agencies must be secure."
May talked with Trump about the issue inside the closed meetings, a senior British government official said.
Trump is already under fire at home for allegedly violating intelligence agreements, following Washington Post reporting that he revealed sensitive information on the Islamic State, obtained from Israel, to the Russian foreign minister and the Russian ambassador to the United States.
In a presidential statement issued while Trump was at the ceremony, he called the Manchester leaks "deeply troubling," vowed to "get to the bottom" of them and called for a full investigation by U.S. agencies, one that could end with prosecutions, he said.
During last year's campaign, Trump called into question the U.S. commitment to NATO's security guarantees, saying he would check a member's defense commitment before coming to its aid. Since then, Cabinet officials have pledged to defend the alliance, but top officials of other NATO allies said that Trump's personal guarantee would eliminate any lingering doubts.
In the Brussels speech, Trump gave no specific commitment to Article 5, the collective-security provision that has been invoked only once — after the 9/11 attacks.
A senior administration official said that "the intent was to deliver a direct message, which he's done before. He's been direct with them in rallies, in speeches. He wanted to give the same message that he's been giving when NATO leaders are present or are not present," the official said, speaking on the condition of anonymity to expand on Trump's remarks. "It's the same message he gave on the campaign trail, it's the same message he gives to the American people, and it's the same message he gives to leaders one-on-one."
Trump began his day in Brussels at a meeting with E.U. leaders Donald Tusk and Jean-Claude Juncker.
"Values and principles first, this is what we — Europe and America — should be saying," Tusk told reporters after the meeting. Tusk, who has previously expressed concern about the new U.S. administration, said he and Trump agreed on counterterrorism but did not see eye to eye on a number of other issues, including climate change, trade and Russia.
Europe has been concerned about Trump's relationship with Russia, particularly over sanctions imposed after its 2014 military involvement in Ukraine and annexation of Crimea. Some U.S. lawmakers have proposed additional sanctions in response to what U.S. intelligence has said was Russia's interference in the U.S. presidential election last year.
National Economic Council Director Gary Cohn, who briefed reporters aboard Air Force One en route to the G-7 meeting, said the president is "looking" at sanctions against Russia. "Right now," he said, "we don't have a position."
At lunch with Trump, Macron repeated France's urging that the United States not pull out of the 2015 Paris climate accord, a decision at which Trump has hinted but which the administration says has not been made.
On this fourth and penultimate stop on Trump's nine-day trip, the first overseas travel of his presidency, Trump did not appear to find the near-adulation he experienced from Arab leaders in Saudi Arabia, and from the Israeli government in Jerusalem.
During those stops, the leaders agreed with Trump's call to concentrate on counterterrorism and economic growth, with no discussion, at least in public, of human and civil rights concerns that had dogged U.S.-Middle East relationships under President Barack Obama. Compared with his clear ebullience and declarations of personal friendship with leaders in the Middle East, Trump appeared standoffish and solitarily glum among his NATO colleagues.
NATO's leaders used the excuse of the vast new headquarters to invite the former real estate mogul for a ribbon-cutting, even though construction on the site — a former military airfield — has not been completed. Beyond its official purpose, however, the meeting was designed to allow Trump and NATO to take the measure of each other.
Some allies have felt the golden word of the president would finalize the message to Russia and others across the NATO border that the United States had their backs.
"At the end of the day, all important decisions are made by the president. And usually the president has a few options on the table," Latvia's state secretary for foreign affairs, Andrejs Pildegovics, said ahead of the dinner meeting. Afterward, he said he still heard an endorsement of the relevance of NATO's traditional mission.
NATO pledged in 2014 that all members will reach the goal of spending at least 2 percent of their gross domestic product on defense within 10 years. Stoltenberg noted Thursday morning that overall spending among members has been up for two years in a row, and he said he anticipated that increases would now speed up as the alliance addresses the terrorism threat.
He also said NATO was ready to join the U.S.-led coalition against the Islamic State — to which all individual members already belong. Among other increased contributions to counterterrorism — which Trump has urged — he said the alliance would step up support of NATO AWACS planes and intelligence-sharing, and provide refueling capabilities.
"We will now establish a new intelligence fusion cell at the headquarters addressing terrorism, including foreign fights," and appoint a special coordinator for NATO's counterterrorism efforts, Stoltenberg said. He called it a "strong political message" as well as a practical one.
Stoltenberg also said NATO would consider increasing its noncombat troop presence in Afghanistan. The Trump administration is reviewing the U.S. presence there, including possibly adding some 3,000 troops to the 8,400 already on the ground and expanding their role, which now consists of assisting Afghan government forces fighting both the Taliban and a local Islamic State presence.