Trump said the U.S. would not "forsake the friends" who came to America's aid after the deadly terrorist attacks. He did not, however, explicitly mention and endorse "Article 5," the mutual assistance clause in the NATO charter that he was widely expected to back publicly for the first time .

The ceremony in Brussels dedicated a memorial to the only time NATO has invoked that automatic defense clause — after the Sept. 11, 2001, attacks — when those same NATO nations came the America's defense by sending troops to Afghanistan and elsewhere.

President Donald Trump stood next to a twisted piece of the World Trade Center and addressed America's NATO allies on Thursday — but he didn't give them the assurance they wanted that the United States will automatically come to their defense.

French President Emmanuel Macron (L) and German Chancellor Angela Merkel (2nd L) speaks as US President Donald Trump (C) arrives next to Greek Prime Minister Alexis Tsipras (R) for a family picture during the NATO (North Atlantic Treaty Organization) summit at the NATO headquarters, in Brussels, on May 25, 2017.

Trump, who won the White House with an anti-global message, has repeatedly bashed NATO and the European Union, which have formed the basis of U.S.-Europe cooperation for decades. Trump's failure to commit to honoring Article 5 if a U.S. ally is attacked has come as Russia has become more militarily assertive.

His speech Thursday likely created more unease for the leaders of NATO countries, said Ivo Daalder, a former U.S. ambassador to the alliance.

"The leaders of NATO were ... hoping to hear those words of affirmation. It didn't happen ... and that was a problem," he told MSNBC.

The White House insisted that Trump's mere presence at the meeting was an endorsement of Article 5. A senior White House official told NBC News that "participation alone" should "be seen as an endorsement of Article 5."

Explicitly endorsing it "would be redundant," the official said.

Hallie Jackson tweet: What's up w/lack of an explicit endorsement of Article 5 from @POTUS? A sr . WH official tells me "participation alone" affirms commitment:

"The mere fact" that allies looked for Trump to publicly reaffirm the commitment to Article 5 "is a testament to their concern about his support for NATO," said Charles Kupchan, a senior fellow at the Council on Foreign Relations and former National Security Council official in the Obama and Clinton administrations.

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