BRUSSELS — Visiting a city he once called a “hellhole” to meet with the leaders of one alliance he threatened to abandon and another whose weakening he cheered, President Donald Trump will address a continent today still reeling from his election and anxious about his support.

Trump traveled this morning to the European Union headquarters in Brussels for meetings with Donald Tusk, the president of the European Council, Jean-Claude Juncker, the president of the European Commission, and other EU officials.

Trump appeared to be greeted warmly by the leaders, despite his past comments publicly cheering the United Kingdom’s vote to leave the EU last summer and slamming the alliance during his transition as “a vehicle for Germany.” Trump has taken a less combative tone since taking office, praising the alliance as “wonderful” and saying a strong Europe is very important to him and the United States.

After meeting with Trump today at the EU, Tusk said he and Trump agreed on the need to combat terrorism but some differences loomed large.

“Some issues remain open, like climate and trade. And I am not 100 percent sure that we can say today — we means Mr. President and myself — that we have a common position, common opinions about Russia,” said Tusk, who said unity needed to be found around values like freedom and human rights and dignity.

“The greatest task today is the consolidation of the whole free world around those values,” he said.

Later in the day, Trump is slated to meet with France’s new president and attend his first meeting of NATO, the decades-long partnership that has become intrinsic to safeguarding the West but has been rattled by the new president’s wavering on honoring its bonds. Trump has mused about pulling out of the pact because he believed other countries were not paying their fair share and he has so far refused to commit to abiding by Article 5, in which member nations vow to come to each other’s defense.

A new strain emerged on the relationship between the U.S. and a key European ally: a British official said today that police in Manchester will stop sharing information about their bombing investigation with the U.S. until they get a guarantee that there will be no more leaks to the news media.

British Prime Minister Theresa May said she plans to push Trump that “intelligence that is shared between our law enforcement agencies must remain secure.” The source of the leak is unclear. Trump did not respond to shouted questions as to whether the UK can trust the U.S. with sensitive material.

But the European capitals that have been shaken by Trump’s doubts may soon find a degree of reassurance. Just like his position on the EU, the president has recently shifted gears, praising NATO’s necessity. Secretary of State Rex Tillerson said Wednesday that “of course” the United States supports Article 5, though Trump still wants other nations to meet their obligation to spend 2 percent of their gross domestic product on defense.

Trump is slated to leave Brussels late today for the final piece of his trip, a two-day stay in Sicily for G-7 meetings.