If reelected, he would end our commitment to the European alliance, reshaping the international order and hobbling American influence in the world.

"I don’t give a shit about NATO.” Thus did former President Donald Trump once express his feelings about America’s oldest and strongest military alliance. Not that this statement, made in the presence of John Bolton, the national security adviser at the time, came as a surprise. Long before he was a political candidate, Trump questioned the value of American alliances. Of Europeans, he once wrote that “their conflicts are not worth American lives. Pulling back from Europe would save this country millions of dollars annually.” NATO, founded in 1949 and supported for three-quarters of a century by Democrats, Republicans, and independents alike, has long been a particular focus of Trump’s ire. As president, Trump threatened to withdraw from NATO many times—including, infamously, at the 2018 NATO summit.

But during Trump’s time in office, the withdrawal never happened. That was because someone was always there to talk him out of it. Bolton says he did; Jim Mattis, John Kelly, Rex Tillerson, Mike Pompeo, and even Mike Pence are thought to have done so too.

But they didn’t change his mind. And if Trump is reelected in 2024, none of those people will be in the White House. All of them have broken with the former president, in some cases dramatically, and there isn’t another pool of Republican analysts who understand Russia and Europe, because most of them either signed statements opposing him in 2016 or criticized him after 2020. In a second term, Trump would be surrounded by people who either share his dislike of American security alliances or don’t know anything about them and don’t care. This time, the ill will that Trump has always felt toward American allies would likely manifest itself in a clear policy change. “The damage he did in his first term was reparable,” Bolton told me. “The damage in the second term would be irreparable.”

Institutionally, and maybe even politically, leaving NATO could be difficult for Trump. As soon as he announced his intentions, a constitutional crisis would ensue. Senate approval is required for U.S. treaties—but the Constitution says nothing about congressional approval for withdrawal from treaties. Recognizing this gap in the law, Democratic Senator Tim Kaine and Republican Senator Marco Rubio introduced legislation, which has already passed the Senate, designed to block any U.S. president from withdrawing from NATO without two-thirds Senate approval or an act of Congress. Kaine told me he feels “confident that the courts would uphold us on that and would not allow a president to unilaterally withdraw,” but there would certainly be a struggle. A public-relations crisis would unfold too. A wide range of people—former supreme allied commanders, former chairmen of the Joint Chiefs of Staff, former presidents, foreign heads of state—will surely rally to make the case for NATO, and very loudly.

But none of that would necessarily matter, because long before Congress convenes to discuss the treaty, the damage will have been done. That’s because NATO’s most important source of influence is not legal or institutional, but psychological: It creates an expectation of collective defense that exists in the mind of anyone who would threaten a member of the alliance. If the Soviet Union never attacked West Germany between 1949 and 1989, that was not because it feared a German response. If Russia has not attacked Poland, the Baltic states, or Romania over the past 18 months, that’s not because Russia fears Poland, the Baltic states, or Romania. The Soviet Union held back, and Russia continues to do so now, because of their firm belief in the American commitment to the defense of those countries.

This deterrent effect doesn’t come just from the NATO treaty, a bare-bones document whose signatories simply agree in Article 5 that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” Deterrence comes from the Kremlin’s conviction that Americans really believe in collective defense, that the U.S. military really is prepared for collective defense, and that the U.S. president really is committed to act if collective security is challenged. Trump could end that conviction with a single speech, a single comment, even a single Truth Social post, and it won’t matter if Congress, the media, and the Republican Party are still arguing about the legality of withdrawing from NATO. Once the commander in chief says “I will not come to an ally’s aid if attacked,” why would anyone fear NATO, regardless of what obligations still exist on paper? And once the Russians, or anyone else, no longer fear a U.S. response to an attack, then the chances that they will carry one out grow higher. If such a scenario seems unlikely, it shouldn’t. Before February 2022, many refused to believe there could ever be a full-scale Russian invasion of Ukraine.

When I asked several people with deep links to NATO to imagine what would happen to Europe, to Ukraine, and even to Taiwan and South Korea if Trump declared his refusal to observe Article 5, all of them agreed that faith in collective defense could evaporate quickly. Alexander Vershbow, a former U.S. ambassador to NATO and a former deputy secretary-general of NATO, pointed out that Trump could pull the American ambassador from his post, prevent diplomats from attending meetings, or stop contributing to the cost of the Brussels headquarters, all before Congress was able to block him: “He wouldn’t be in any way legally constrained from doing that.” Closing American bases in Europe and transferring thousands of soldiers would take longer, of course, but all of the political bodies in the alliance would nevertheless have to change the way they operate overnight. James Goldgeier, an international-relations professor at American University and the author of several books on NATO, thinks the result would be chaotic. “It’s not like you can say, ‘Okay, now we have another plan for how to deal with this,’ ” he told me. There is no alternative leadership available, no alternative source of command-and-control systems, no alternative space weapons, not even an alternative supply of ammunition. Europe would immediately be exposed to a possible Russian attack for which it is not prepared, and for which it would not be prepared for many years.

without nato, and without an American commitment to European security, supplies for Ukraine would also dry up. The prospect of America leaving NATO would force many European countries to keep their military resources at home; after all, they might soon face invasion as well. The Ukrainians would begin to run out of ammunition quite quickly. The Russian conquest of all of Ukraine—still President Vladimir Putin’s goal—would become thinkable once again. Ukrainian military logistics would become much harder, because the Russians could bomb airports and other supply hubs in Poland and Romania. They have already come very close: At least one Russian missile accidentally struck Poland, and Russian strikes have hit the Romanian-Ukrainian border. Early in the war, the Russians deliberately attacked a base in western Ukraine, very near the Polish border, where foreign soldiers were known to be training. If the Russians begin to target bases inside Poland itself, the logistics of arming Ukraine become impossible.

This change would immediately reverberate beyond Europe. Once Trump has made clear that he no longer supports NATO, all of America’s other security alliances would be in jeopardy as well. Taiwan, South Korea, Japan, and even Israel would figure they can no longer count on automatic American support. The end of NATO might not affect them directly, but its demise would signal that everyone, everywhere, has to assume the United States is no longer a reliable ally.

Over time, all of America’s allies would begin to hedge. Many European countries would cozy up to Russia. Many Asian countries would calculate that, as Kaine puts it, “I guess we need to get closer to China, just as a matter of self-preservation.” To avoid invasion, pragmatic leaders near China or Russia might begin to take more seriously the commercial and political demands from the world’s second- and third-largest military powers, respectively. At the same time, many political parties and heads of state (both in and out of power) backed by Russia and China—or Iran, Venezuela, Cuba—would have a compelling new argument in favor of autocratic methods and tactics: America, a country whose image has already been severely damaged by Trump and Trumpism, would be seen to be retreating. Over time, American economic influence would decline too. Trade agreements and financial arrangements would change, which would have an impact on American companies and eventually the U.S. economy.

If Trump is reelected, Americans will be so consumed by the drama of their own failing institutions that, for a long while, most won’t note the problems caused by the shifting international order. Lithuania’s and South Korea’s troubles would seem distant, irrelevant. The end of American influence would probably unfold in relative obscurity. By the time people here realize how much has changed, it will be too late.