Europe and the United States are on the verge of the most momentous conscious uncoupling in international relations in decades. Since 1949, NATO has been the one constant in world security. Initially an alliance among the United States, Canada, and 10 countries in Western Europe, NATO won the Cold War and has since expanded to include almost all of Europe. It has been the single most successful security grouping in modern global history. It also might collapse by 2025.

The cause of this collapse would be the profound difference in outlook between the Republican Party’s populist wing—which is led by Donald Trump but now clearly makes up the majority of the GOP—and the existential security concerns of much of Europe. The immediate catalyst for the collapse would be the war in Ukraine. When the dominant faction within one of the two major American political parties can’t see the point in helping a democracy-minded country fight off Russian invaders, that suggests that the center of the political spectrum has shifted in ways that will render the U.S. a less reliable ally to Europe. The latter should prepare accordingly.

[Read: What America’s great unwinding would mean for the world]

The past few weeks have revealed that Trump’s pro-Russian, anti-NATO outlook isn’t just a brief interlude in Republican politics; suspicion of American involvement in supporting Ukraine is now the consensus of the party’s populist heart. During last week’s GOP presidential debate, Ron DeSantis and Vivek Ramaswamy—the two candidates most intent on appealing to the party’s new Trumpist base—both argued against more aid for Ukraine. DeSantis did so softly, by vowing to make any more aid conditional on greater European assistance and saying he’d rather send troops to the U.S.-Mexico border. Ramaswamy was more strident: He described the current situation as “disastrous” and called for a complete and immediate cessation of U.S. support for Ukraine. Ramaswamy later went even further, basically saying that Ukraine should be cut up; Vladimir Putin would get to keep a large part of the country. Trump did not take part in the debate, but he has previously downplayed America’s interest in an Ukrainian victory and has seemed to favor territorial concessions by Ukraine to Russia. He, DeSantis, and Ramaswamy are all playing to the same voters—who, polls suggest, make up about three-quarters of the Republican electorate.

Another bellwether is the Heritage Foundation, a prominent conservative think tank that has played an outsize role in GOP policy circles since the Reagan years. Before Russia launched its full-scale invasion, in February 2022, Heritage had been on the hawkish wing of the Republican Party, even publishing a call for Ukraine to be accepted into NATO. More recently, Heritage officials have called for halting aid until the Biden administration produces a plan to end the war—which is an impossible goal unless Russia agrees. Demagogues on the right are taking Putin’s side even more overtly. The talk-show host Tucker Carlson, for instance, in a August address in Budapest, maintained that anti-Christian bias motivated American opposition to Russia.

Such claims are ridiculous, not least because Russia is one of the least religious societies on Earth. But the growing sentiment on the American right against supporting Ukraine represents an extraordinary challenge to the future of NATO. European states have been moving in the opposite direction: As evidence mounts of Russian atrocities in Ukraine, and Russia shows itself willing to commit almost any crime in its desire to seize the territory (and people) of an independent, internationally recognized country next door, many European countries (particularly many of those close to Russia) have come to see this war as one that directly challenges their future. If Putin were to keep large pieces of Ukraine, that outcome would represent not peace, but a form of perma-war, in which a revanchist Russia would have established its ability to seize the land of its neighbors.

Even if Joe Biden wins reelection, Republican control of the House, Senate, or both could substantially weaken U.S. support for the Ukrainian effort. And if Trump or one of his imitators wins the presidency in November 2024, Europe could find itself faced with a new American administration that will halt all support for Ukraine.

Such a move would make the U.S. itself an obstacle to a long-term free and stable Europe. It would split the Atlantic alliance, and European states have not prepared themselves for that possibility.

[From the July 1861 issue: The United States and Europe]

The reality is that, for many years, Europe has largely slipstreamed behind the U.S. on security matters. This has provided real benefits to the U.S., by solidifying American leadership in the world’s most important strategic grouping while allowing European states to spend far less on defense than they would otherwise have to. The differential also means that Europe, on its own, lacks the breadth and depth of U.S. military capabilities.

The Western aid given to Ukraine highlights the difference between the two sides. Over the past year, leaders in Europe have been more insistent than Washington about the need to provide powerful, advanced equipment to Kyiv, but their reliance on European-made systems has limited their ability to deliver. The U.K. and France have supplied long-range cruise missiles—known as Storm Shadow in Britain and SCALP in France—that they jointly developed, but the two countries have substantially less equipment to spare than the U.S. does. Although the greatest amount of military aid has come from the U.S., the Biden administration has slow-walked the transfer of more advanced material such as Abrams tanks (which have yet to appear on the battlefield in Ukraine), F-16 fighter jets (which won’t show up until 2024), and Army Tactical Missile Systems equipment (for which the administration continues to make spurious arguments for withholding).

What leaders in Europe have to face, as a pro-Russia, anti-Ukraine position solidifies in the Republican Party, is the prospect of having to do most of the heavy lifting to help Ukraine win the war. That is no small task. Europe would have to expand its manufacturing capacities both for ammunition and other nuts-and-bolts military needs and for the more advanced systems, such as long-range missiles, that it would have to supply on its own.

If the United States simply abandons Ukraine a year and a half from now, there is no way whatsoever that Europe could make up for the loss of aid. But European governments would have to come up with ways to ameliorate that withdrawal. This would require tact and skill—and the preparations would have to start soon. European military officials need to quietly ask their Ukrainian counterparts what the latter would need that the former could supply if American assistance wanes, and then start figuring out how to ramp up production. Such planning would also allow European militaries to start thinking about how they, alone, could defend Europe against Russian aggression. For years, military planners on the continent have debated whether, in the interest of maximizing overall security, individual European nations should specialize their military operations; instead of most states operating a small army, navy, and air force of their own, each would concentrate on the roles that best fit its location, population, and production base, and then rely on other states with complementary capacities. A continent-wide effort to accelerate weapons production for Ukraine would force the question.

Without committing itself to such comprehensive military planning, Europe could also find itself in an internal diplomatic crisis. Countries in the east (such as Poland and Romania) and North (such as the Baltic and Scandinavian nations) are desperate to see Russia defeated. But if Europe fails to embark soon on a unified, collective military-production plan, countries in the west and south that feel less threatened by Russian aggression might be inclined to follow the lead of a new American administration that backs away from Ukraine and tries to cut a deal with Russia. The result could be a legacy of bitterness and distrust at best, and a permanent fracturing of European cooperation at worst.

Hopefully these scenarios won’t materialize. The election of a pro-NATO and pro-Ukraine U.S. president in 2024 should be enough to see Ukraine through to a military victory and peace deal (which would involve Ukraine’s admission into NATO), leading to security on the continent. But that possibility doesn’t absolve European leaders of the obligation to plan for an alternate reality in which an American administration scuttles NATO and seeks a rapprochement with Putin, despite Russia’s genocidal crimes against a European state. If the Europeans don’t start planning for the worst-case scenario, they will have no one to blame but themselves.

All the latest developments from the war in Ukraine.