He then segued to a claim that he had already “raised” an additional $44 billion from NATO members and there was more to come, followed by a riff on how immigration was “destroying the culture of Europe.”

The answer to Mr. Carlson’s and Mr. Trump’s question on why defend Montenegro from attack is, of course, Article 5 of the NATO treaty, the central tenet that requires every member to come to the aid of any ally under attack. That does not mean NATO would have to pile in if Montenegro aggressively assaulted, say, Serbia, since the article is triggered only if a member is attacked. It has been invoked only once: not in the Balkans, but in support of the United States after 9/11.

The broader question, whether NATO is needed any longer in the post-Communist world, has been extensively debated over the past quarter century and answered in the affirmative, as a model of collective security and as a trans-Atlantic bond. It is clear that Mr. Trump, in his zero-sum view of global forces, knows nothing of this history or debate. A larger question is whether he is aware that his friend Mr. Putin strenuously opposed Montenegro’s joining NATO, and that Russia is suspected of being behind a failed 2016 plot to overthrow its government and assassinate its prime minister.

In any case, the response from Montenegro was, indeed, aggressive. “He’s the strangest president in the history of the United States,” Ranko Krivokapic, a former president of the Montenegro Parliament, told the BBC. Reactions at home were less generous. “It is not just that the president throws Montenegro under the bus; he makes the U.S. commitment to NATO conditional and makes clear his discomfort w/Article 5 and collective security, the core of the alliance,” tweeted Richard Haass, the president of the Council on Foreign Relations. Senator John McCain, the Arizona Republican, wrote on Twitter, “By attacking Montenegro & questioning our obligations under NATO, the President is playing right into Putin’s hands.”

Mr. Trump might get his staff to find another missing double negative to roll back what he said. A far better job for his people would be to compel him to understand that petty and uninformed whims from his high office are incomparably more dangerous to America’s security, and the world’s, than to a tiny Balkan country.

Follow The New York Times Opinion section on Facebook and Twitter (@NYTOpinion), and sign up for the Opinion Today newsletter.