President Donald Trump’s trip to Europe last week produced such a staggering volume of news that it is getting hard to keep up. Trump accused Germany of being “a captive of Russia,” warned nato allies that the United States “might go it alone” if they didn’t increase spending immediately, accused the British prime minister of “wrecking Brexit,” and met one-on-one with the Russian president, after which he appeared to express more confidence in Vladimir Putin than in his own intelligence services. White House aides have since worked feverishly—and unconvincingly—to convince the world that Trump didn’t actually mean what he said, or didn’t say what he meant. At the same time, they have introduced new controversies—for example, by suggesting that the United States would consider Putin’s outrageous idea that Russian officials be allowed to interrogate the former U.S. ambassador to Russia Michael McFaul for alleged misdeeds in Russia.

The stories are so numerous and head spinning that potentially lost in the shuffle is what in the long run could be the most consequential development of all: that Trump has dealt a potentially fatal blow to America’s most effective defense alliance, nato, and may have made Europe safe again for war.

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 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.”

The World This Week A weekly digest of the latest from CFR on the biggest foreign policy stories of the week, featuring briefs, opinions, and explainers. Every Friday. View all newsletters >

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 their own defense.

Since the nato treaty itself does not legally require allies to come to each other’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 members 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.

The Montenegro issue is a symptom of a much larger problem, which is that Trump doesn’t believe in alliances generally, and nato in particular. When he said during the election campaign that nato was “obsolete,” he meant it. His singular and obsessive focus on relative contributions to nato defense spending reflects not just a shocking misunderstanding of how nato works—allies do not pay the United States to defend them or “owe” Washington anything—but a deeper skepticism about the very concept of an alliance. While past presidents complained about Europeans not paying their fair share, they all recognized that even if Americans contributed more than others, natoserved American interests because the benefits of peace and prosperity in Europe vastly exceeded the costs of U.S. defense spending—much of which was necessary for tasks that had nothing to do with nato. Trump’s argument in the Carlson interview that the United States pays “90 percent” of the costs of defending Europe reflects either deep ignorance about nato and defense budgeting or, worse, a deliberate attempt to mislead his supporters into turning against the alliance. In reality, total U.S. military spending represents just over 70 percent of all nato countries’ defense spending, only about 15 percent of which goes to defend Europe.

Trump’s attacks on nato allies and his questioning of the principle of collective defense are even more damaging in the wake of the Helsinki summit, during which he showed himself to be shockingly deferential to Russia and Putin. While his statements on Moscow’s election interference got the most press, his inability to condemn Russia for its invasion of Ukraine, bombing in Syria, and poisoning of people on British soil were at least as noteworthy. If the U.S. president is afraid of stressing differences with Russia at a press conference, could allies ever count on him to confront Russia on a potential European battlefield?

Trump, of course, has said many negative things about nato in the past, and he’s made a habit of saying nice things about Putin. So why is this time different? In part because the accumulated weight of his statements and actions now make clear—to America’s allies in Europe as well as to its adversary in Moscow—that neither can count on Trump to come to the defense of American allies. Putin has already demonstrated that he has little problem with using force, including in Europe, and including to change borders through violent means. Since neither were nato members, Putin felt free to invade Georgia in 2008 and Ukraine in 2014. But Trump has now questioned the value of nato membership to the point that Putin may no longer see the use of force against other allies as beyond the pale. What about the Baltic states, which were part of the Soviet Union for 50 years and have significant ethnic Russian and Russian-speaking populations? Would Putin dare challenge nato there? Or Poland? Where would it stop?

These aren’t far-fetched scenarios. They are very real questions that people in nato capitals, and in Moscow, are naturally asking. As former Polish Foreign Minister Radosław Sikorski wrote on July 17 in The Washington Post, “After last week, we have to face the reality that we do not know—because even the U.S. State Department, the Pentagon and Trump’s own National Security Council do not know—what Trump would do in a security crisis with Russia.” As one former senior U.S. official persuasively argued last summer when Trump questioned the need to defend nato against Russia, “The whole point of having a strong nato alliance is not to engage in warfare with Russia, it’s to deter the Russians from even thinking about using military force to achieve their objectives in Europe and if … the leader of the nato alliance … shows weakness or uncertainty it destroys those structures of deterrence that we worked for more than 65 years to build up.” That commentator was John Bolton, now the president’s national-security adviser.

Alliances are built on trust—on the belief that another country will come to your defense if it is attacked. Trump’s statements are undermining that trust, possibly with longterm consequences. While both houses of Congress voted overwhelmingly last week to reaffirm the U.S. commitment to nato, Trump’s relentless anti-alliance rhetoric has begun to have a disturbing impact among the Republican base. As many Republicans (38 percent) think America should leave nato as those who think it should remain a member, and two-thirds of registered Republicans now believe the United States should refuse to uphold its nato treaty commitments if allies don’t spend more on defense. This represents a 15 percent increase since last year.

nato still matters. It has kept the peace in Europe since World War II at a fraction of the cost of another war or building the armies that would have made individual countries feel safe. That peace enabled Europeans to rebuild from the devastation of two wars, and to build the most prosperous and democratic community in history. By focusing on becoming rich, Europe emerged as America’s largest trading, security, and democratic partner in the world. Without nato, European countries will turn inward, focus on their own defense and security, and seek safety either in appeasement of Moscow or through confrontation that could lead to a conflict for which they are unprepared. These possibilities aren’t nightmares; they quickly can become reality once you question the fundamentals of the most successful alliance in history. For a president who likes to win, that could prove a historic, deadly loss.

Originally published as “Trump’s Biggest Gift to Putin” at The Atlantic, July 19, 2018.