NATO leaders weren’t wrong to be unnerved by Donald Trump’s hair-raising performance in Brussels last week—an event that one attendee described to me as P.T.S.D.-inducing for America’s allies. The NATO alliance, after all, is premised on the credible threat of military action if any individual member is attacked—and Trump continues to give every indication that Europe, as far as he is concerned, is on its own. Even after the brutal response to his servile performance in Helsinki, Trump could not pretend to care about the alliance, telling Fox News host Tucker Carlson in an interview Tuesday night that he wouldn’t want to defend “tiny” Montenegro—deepening fears that he might not uphold Article 5 of the NATO charter, the principle that an attack on one NATO country is an attack on all, if Russia invaded.

“Membership in NATO obligates the members to defend any other member that’s attacked,” Carlson said to Trump. “So let’s say Montenegro, which joined last year, is attacked. Why should my son go to Montenegro to defend it from attack?” In response, Trump told Carlson that he sympathized with his view. “I’ve asked the same question. Montenegro is a tiny country with very strong people,” the president said. “They’re very aggressive people. They may get aggressive, and congratulations you’re in World War III.”

The defense of Montenegro is not a frivolous concern. In October of 2016, a group of Serbians and Montenegrins were arrested as part of an alleged coup d’état involving two Russian nationals. Last July, several members of the group were charged with attempting to kill then-Prime Minister Milo Djukanovic and bring a pro-Russian party to power, and to prevent Montenegro from joining NATO. (The Kremlin has denied these allegations.) The accession of the small Southeastern European sovereignty into the NATO alliance—which President Trump approved at the time—was hailed as a victory for pro-Western forces over Moscow’s political meddling in its former sphere of influence.

By casting doubt on the founding principle of NATO, Trump again seemed to side with America’s adversaries over its allies. As Nicholas Burns, who served as the U.S. ambassador to NATO under George W. Bush, explained to me last week, the NATO alliance is only effective as a deterrent against Russian aggression if President Vladimir Putin believes that the U.S. and the alliance’s 28 other members will respect Article 5. “The big danger here is that Putin needs to believe that Trump is the leader of NATO, is willing to defend NATO countries that could be threatened by Russia,” he told me. “That is what deterrence is all about, that is how deterrence works, right? The other guy, in this case the adversarial power Putin, has to believe in his heart of hearts that he cannot take aggressive measures toward NATO countries because Trump and the other leaders would stand up to him.”

Ironically, the only time NATO’s common defense doctrine has been invoked was after the 2001 terrorist attacks on the United States—a fact underscored by Trump’s failure to commit to Article 5 last year, as he stood before a 9/11 memorial. He eventually relented amid pressure from Congress and members of his Cabinet, explicitly reaffirming the U.S.’s commitment to the clause, but there have been lingering doubts, which were heightened following the NATO summit in Brussels last week. “[Other NATO members] don’t know whether to be relieved that the alliance is intact—and actually has a lot of strength behind it with the real decisions that came out of the summit,” Damon Wilson, the executive vice president of the Atlantic Council, told me on Thursday, “or whether they need to really start deciding that they need to hedge on the United States.”

That Carlson zeroed in on Montenegro, of the more than two-dozen countries that make up the alliance, is noteworthy. While the small country reportedly contributes 1.5 percent of its gross domestic product to defense spending—less than the agreed-upon 2 percent contribution—it supplies a greater number of troops to the NATO operation in Afghanistan than the U.S. on a per capita basis. Nor was Trump’s characterization of Montenegro’s population as “aggressive” without irony—at last year’s NATO summit, the president infamously pushed aside Dusko Markovic, the prime minister of the Balkan country, to get to the front of a group photo.