President Donald Trump's first trip to a NATO summit Thursday did not sit well with former ambassadors to the alliance.
"I do think Trump's visit to NATO was the least effective of any American president since 1949," Nicholas Burns, who served as ambassador to the 28-member defense alliance under President George W. Bush, tweeted Friday. NATO came into existence in 1949.
In Brussels, Trump admonished members of the alliance for not paying their "fair share" for defense. The president failed to publicly endorse "Article 5," the NATO mutual assistance clause that he was widely expected to back publicly for the first time.
Trump was silent on Article 5 while standing next to a twisted metal piece of the World Trade Center, a new memorial to the Sept. 11, 2001, attacks.
NATO states came to America's aid after 9/11 — the only time in the alliance's history that it has invoked Article 5. About a third of the NATO soldiers killed in Afghanistan have come from countries other than the United States.
Trump's speech Thursday likely created more unease for the leaders of NATO countries, said Ivo Daalder, another 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 on Thursday.
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. His failure to commit to honoring Article 5 if a U.S. ally is attacked has come as Russia has become more militarily assertive.
The White House insisted Thursday 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.
Another official who served in the Bush administration applauded Trump's call for more defense spending from allies. Former Defense Secretary Donald Rumsfeld said it was important for Trump to "look them in the eye and talk to them" about hitting their defense spending target.
"Those countries, for the most part, are not meeting the NATO target. And they should, and someone needs to tell them that, and I'm glad he did it," Rumsfeld told MSNBC on Friday.
Rumsfeld called Trump's posture toward allies "unusual" but contended that "you have to cut him a little slack" because he still has a small foreign policy team.
The White House did not respond to a request for comment.