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.

U.S. President Donald Trump poses with fellow world leaders during a NATO summit in Brussels, Belgium, May 25, 2017. Jonathan Ernst | Reuters

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. Leaders including Germany's Angela Merkel and France's Emmanuel Macron looked on uncomfortably and exchanged private asides as Trump spoke Thursday. 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.

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