Europe is feeling a bit abandoned these days. Used to regular reassurances from Washington on its commitment to transatlantic security, European leaders are now shifting their defense strategies in response to mixed signals from President Donald Trump on the United States’ role in NATO. Even Secretary of Defense James Mattis has come down hard on the alliance, demanding that Europe “show its support for our common defense” or else Washington would alter the nature of their partnership. Worried Europeans, however, may be slightly comforted by U.S. Secretary of State Rex Tillerson’s decision to attend a NATO foreign ministers’ meeting on March 31, after originally planning to miss the gathering.

Fear that the United States might not live up to its promises, especially to the alliance’s eastern members, has led to a modest uptick in European conventional military outlays, which rose 0.5 percent in 2015 and 3.8 percent in 2016 year in real terms. This is not enough to transform the balance of power, but it is a notable shift. Growing alarm that Europe might end up on its own against Russia has also led to the previously unmentionable idea of developing Europe-wide nuclear deterrence.

Moscow’s 2014 annexation of Crimea from Ukraine and its support for separatists in the Donbass have reminded NATO’s old Europe members that the organization is a military alliance, not an international social club. When NATO extended membership to the Baltic states, few policymakers believed the alliance’s Article 5 promise of mutual security would one day be invoked. Today, they are wondering how they would defend the vulnerable countries without the United States.

Last year, the Atlantic Council’s Matthew Kroenig contended that “NATO must be able to deter a Russian nuclear attack, counter the nuclear coercion inherent in Russia’s hybrid warfare strategy, and assure NATO members that the Alliance is prepared to defend them.” He concluded that this would “require strengthening NATO’s existing nuclear deterrence strategy and capabilities.”

Russia’s potential use of nuclear weapons is a particularly vexing challenge for Europe to confront on its own. Based on Moscow’s battlefield exercises, threats to use nuclear weapons, and other factors, it is commonly believed that Russia’s conventional military weakness has caused it to lower its threshold for deploying nukes. Although NATO members France and the United Kingdom developed their own nuclear forces during the Cold War, they never planned to face down the Soviet Union in order to protect West Germany. Paris and London seem no more prepared today to deploy their nukes to defend the alliance’s eastern members. The presumption has always been that the United States would make the decisive nuclear move against Russia. 

On second thought, however, what American president would risk triggering a nuclear conflict in defense of Estonia, Latvia, or Lithuania? NATO was originally created to prevent the Soviet Union from dominating Eurasia. For that strategic objective, Washington was willing to pay a high price. 

In contrast, Moscow’s conquest of the Baltics—which had been part of the Russian Empire and the U.S.S.R. after only a short period of independence in between—would be horrible for them but not particularly threatening to the United States or even the rest of Europe. In fact, the Baltics were inducted into NATO only because no one believed it would be necessary to defend them. That means that even a solemn U.S. promise to protect them is, in reality, unreliable.

This raises the question of whether Europe should create its own nuclear deterrent. The challenges of such an approach are obvious. Neither France nor the United Kingdom seems inclined to yield control over its arsenals. And it is unlikely that a nuclear Germany, or any other European state, would threaten Moscow over a territorial incursion in Poland, let alone Estonia.

Roderich Kiesewetter, who holds a lead foreign policy role in the Bundestag, has proposed relying on France and the United Kingdom while financing a larger arsenal through a joint European military budget. Such European weapons could substitute for or supplement American bombs. Similarly, Berthold Kohler, publisher of the Frankfurter Allgemeine Zeitung, speculated that Germany could augment the French and British arsenals in order to confront Russia. Jaroslaw Kaczynski, former prime minister and chairman of Poland’s nationalistic Law and Justice Party, suggested that Europe develop a much larger arsenal to match Russia’s.

Although Ulrich Kühn of the Carnegie Endowment for International Peace has rightly noted it would be far too expensive for Europe to match Russia’s store of 2,000 to 3,000 tactical nuclear weapons, only a deterrent, not equivalent, force is needed. “It’s not a question of numbers,” Kiesewetter has written. “The reassurance and deterrence comes from the existence of the weapons and their deployability.”

Michael Gahler, a member of the European Parliament, is staunchly against a nuclear deterrent, arguing that “this is a debate that should not be started.” His worry, among those of other European leaders, is the impact that European nuclear deterrence would have on nonproliferation. But the latter is a worthwhile objective, not a religious doctrine. Europe should be able to defend itself against Russia, especially since the United States may tire of holding a nuclear umbrella over the continent. 

Europe is also the most obvious place for Washington to close at least one of its nuclear umbrellas. None of the United States’ Asian allies possesses nuclear weapons, and their development would have unpredictable regional effects and be more likely to trigger proliferation. Further, Asia lacks a regional organization such as NATO that could manage a broader, more stable deterrent. Europe, in contrast, already contains two nuclear powers, and the European Union, despite its challenges, could provide a possible continental defense framework. More important, the U.S. nuclear umbrella makes sense only as long as it doesn’t rain. There is no reason for the United States to risk its own security when Russia is no longer an ideological, global competitor and NATO extends up to Russia’s borders.

Most Europeans, including Kiesewetter, still hope that Washington will maintain its role in transatlantic defense, but such a policy runs contrary to fundamental U.S. interests. Seven decades after World War II, circumstances have changed dramatically; so should the United States’ approach to European security.

Although the Trump administration might not seem the most capable of thinking creatively about U.S. foreign policy, the president’s willingness to challenge conventional wisdom creates new opportunities for doing so. It is time for a reconsideration of who should defend Europe, and how.