On the fourth leg of a grueling overseas trip, President Trump lectured NATO allies in Brussels on not spending enough for collective defense, and declined to plainly endorse Article 5 of the alliance's founding treaty, which states that an attack on any member is an attack on all.
For some of the European leaders, testing Trump seemed to be as important as finding common ground with him, amid anxiety about their relationship with a leader who had dismissed the alliance as "obsolete" and called the Belgian capital a "hellhole" after a terrorist attack. Earlier, Trump met with Donald Tusk, the president of the European Council, and Jean-Claude Juncker, the president of the European Commission. Mr. Tusk said afterward that there were differences of opinion over Russia, but that when it came to Ukraine, "it seems that we were on the same line."
Analysts said expectations were low that Trump and European leaders would agree on issues like climate change, trade and terrorism. (He also shared an eyebrow-raising handshake with President Emmanuel Macron of France.)
In the wake of Trump's suggestions that the alliance was not doing enough against terrorism, NATO announced that it would formally join the fight against the Islamic State. If there was any real drama over Trump's visit, it concerned whether he would go off script on the question of Article 5. He had been expected to explicitly endorse the principle in a speech when he unveiled a Sept. 11 memorial — a piece of twisted metal from the World Trade Center — outside NATO's new building.
But in his speech before the leaders, including Chancellor Angela Merkel of Germany, he did not explicitly do so. Instead, Trump offered a vague promise to "never forsake the friends that stood by our side" in the aftermath of the Sept. 11 attacks — a pledge that White House officials later said was an affirmation of mutual defense.
The only time NATO has invoked Article 5 was to defend the United States after the Sept. 11 attacks. More than 1,000 non-American soldiers from NATO countries have died in Afghanistan in the name of Article 5.
The leaders had also wanted him to say something critical about Russia and its annexation of Crimea, but Mr. Trump has been pretty quiet on that topic, too. What he has been vocal about is pressing NATO allies to pay what he considers their fair share of the alliance's running costs. Secretary of State Rex Tillerson, traveling with Trump to Brussels, said the president would have blunt words for the leaders of other NATO members on that issue.
And he did, criticizing the other leaders assembled behind him for not contributing 2 percent of their gross domestic product to their defense, as the allied nations had agreed.
When Tusk emerged from an earlier meeting with Trump and Juncker, there were clear signs that they had differences of opinion over Russia.
"Some issues remained open, like climate and trade," Tusk said shortly after the meeting at European Union headquarters here. "And I am not 100 percent sure that we can say today — 'we' means President and myself — that we have a common position, common opinion, about Russia," he added.