In a post-summit interview Monday night with Fox News’s Tucker Carlson—one of the few commentators anywhere on the political spectrum who wasn’t appalled by Trump’s actions in Helsinki—Trump took subservience to Moscow one step further and challenged the very premise on which NATO is built, in words that eerily echoed long-standing Russian talking points. When Carlson asked why his own son should be prepared to defend the NATO member Montenegro from an attack, the U.S. president responded with apparent delight at finding someone who seemed to think about the issue as he did. “I understand what you’re saying,” Trump responded in a singsong voice that indicated his exasperation. “I’ve asked the same question!” To explain his concern, he went on to note that the Montenegrins are “very aggressive people,” and that “they may get aggressive and congratulations, you’re in World War III.”

Setting aside the irony of Trump calling the Montenegrins aggressive after famously shoving that country’s prime minister out of his camera shot at a previous NATO meeting, Trump’s comments compose nothing short of an existential threat to NATO’s credibility—and an unprecedented gift to Moscow. Since NATO’s founding in 1949 its bedrock principle has been that an attack on one ally would be considered an attack on all. By deterring war, this simple premise has helped produce the longest period of peace in Europe for centuries, while saving vast amounts of money by sparing individual allies the need to spend what would be necessary if they had to rely only on themselves for defense.

Since the NATO treaty itself does not legally require allies to come to one another’s defense (the U.S. Senate would never have agreed to that), its effectiveness has always depended on the U.S. president’s ability to convince allies and adversaries alike that he would actually choose to do so. And whereas all previous presidents did everything they could—from solemn pledges of solidarity to increased military, even nuclear, deployments to Europe—to make that pledge credible in the face of Russian attempts to undermine it, Trump has just publicly announced that he cannot be counted on to back it up.

To give Trump the benefit of the doubt, as White House spinners no doubt will try to do, you could say that Trump, by referencing Montenegrin “aggression,” was merely saying that the U.S. should not have to defend a NATO member if that country itself instigated a conflict. But that argument is a slippery slope, and in fact far more likely to apply to the United States than to any other member of NATO. After 9/11, the only time in NATO’s history when the Article 5 defense guarantee has actually been invoked, the allies did not blame the attack on “aggressive” U.S. policies abroad but simply asked: “How can we help?” Soon thereafter, NATO allies sent tens of thousands of troops to fight alongside Americans in Afghanistan. They spent tens of billions of dollars and suffered more than 1,000 casualties in defense of an ally. Qualifying and conditioning the notion of NATO’s defense guarantee is a major step on the path to abandoning it.