Within hours of a public showdown at the White House between U.S. President Donald Trump and his Ukrainian counterpart, Volodymyr Zelensky, in February, a Ukrainian banker started a fundraising campaign to buy nuclear weapons. Despite the privations inflicted by the war, Ukrainians donated as much as they could and gathered more than half a million dollars before he declared it was meant in jest and redirected the fund toward the purchase of drones.

Oleg Horohovsky, the co-owner of Monobank and a businessman, later said it was meant to “let off steam.” But the speed with which people responded to the joke nonetheless expressed a sense of insecurity that Ukrainians may have felt as their president was berated in Washington and backing from a key ally in an existential war against Russia seemed to have all but vanished.

Ukraine gave up its nukes in 1994 in exchange for assurances from the United States above all that its sovereignty was guaranteed. But as the Trump administration abandons previous promises, Ukraine isn’t the only one feeling insecure and isolated. There are growing fears among Washington’s European allies that they may be left with the French and British nuclear deterrent, if the United States withdraws conventional troops and assets from Europe as it pivots to the Indo-Pacific.

Nuclear weapons are the biggest taboo in Europe, where even whispering about them—and in countries such as Germany, even about the civilian use of nuclear power—is highly controversial. Yet in late February, Friedrich Merz, then the likely next German chancellor, said it was imperative he discussed “whether nuclear sharing, or at least nuclear security from the U.K. and France,” could also apply to Germany. He rushed off to France soon after winning the election, and weeks later French President Emmanuel Macron announced the storage of modern nuclear missiles at an air base less than 150 miles from the German border.

“Our country and our continent must continue to defend, equip, and prepare itself if we want to avoid war,” Macron warned in March.

Despite the signaling, there are concerns that the French and British deterrent is highly insufficient, both in overall numbers and diversity, to effectively dissuade Russia. It was designed merely to complement the U.S. nuclear deterrent, not replace it.

Washington has more than a hundred B61 gravity bombs deployed across the continent in Belgium, Germany, Italy, the Netherlands, and elsewhere. The total U.S. inventory is above 5,000, roughly matching Russia’s arsenal. In comparison, France and Britain, Europe’s only nuclear-armed states, possess a little more than 500 nuclear warheads combined.

Moreover, there are doctrinal challenges. The French nuclear doctrine limits the use of nukes to only if France came under attack. In comparison, Britain has declared that its deterrent extends to European allies, but the British nuclear deterrent itself is dependent on the United States for Trident missiles aboard four Vanguard-class submarines, since the missiles are leased from Lockheed Martin.

Rafael Loss, a fellow with the European Council on Foreign Relations, said it would take more than a decade before the French and British can add more warheads and nuclear capabilities—if they find the political consensus at home and among willing European countries. Then there is the other concern: raising hundreds of billions of dollars in defense investments.

“The cycle to build a nuclear arsenal is even longer than conventional weapons, and while there is no exact estimate, a credible French and British deterrent would require hundreds of billions of euros, rather than tens of billions,” he said. “Some are arguing, for instance in Germany, that Germany could pay the French to extend and enhance its deterrent, but there are questions over who would have control.” Domestic politics and the rise of far-right political groups, which are often seen as Russia-friendly, are also concerns. “First, the Europeans need to build trust among themselves—trust that after French presidential elections in 2027, [the next government] won’t fold up France’s nuclear umbrella like Trump is potentially folding up now,” Loss added.

It is unclear whether the United States will withdraw its troops and other conventional capabilities, but according to leaked documents reported in the Washington Post in March, that’s what U.S. Defense Secretary Pete Hegseth seems to have advocated. The United States will “assume risk in other theaters” and direct personnel as well as other conventional capabilities to deal with China, which the Trump government sees as a bigger threat.

NATO chief Mark Rutte has consistently tried to play down trans-Atlantic tensions, and member states have adopted a policy to manage Trump—at least for as long as they can as they try to fill the capability gaps in the conventional domain. But few doubt that U.S. support to Europe is a thing of the past and that the continent has to buck up and manage its security on its own.

Astrid Chevreuil, a visiting fellow at the Center for Strategic and International Studies, said the credibility of the U.S. nuclear deterrent is under question. “Usually when we think about a nuclear deterrent, there are three aspects to it: military, technical, and political,” she told Foreign Policy over the phone. “The military capability as yet is not under question,” since the United States hasn’t said it will withdraw its nuclear bombs, “and there is no debate over the technical aspect, as systems are effective,” she said. “But the political credibility has indeed come under question—that is, the willingness of the country that owns the bombs to use them.”

“It’s the president or the prime minister of a country who decides whether to use a nuke or not,” and the belief that the United States will, not just to protect itself but also its allies, is a significant part of deterrent psychology. “But the Trump administration has sown doubts,” Chevreuil added.

Experts say that even if the United States withdrew its soldiers, it would still weaken the protections offered by its nuclear deterrent. Any withdrawal of U.S. troops would indicate to its adversaries that the United States isn’t likely to use the nukes in response to an attack on an ally nation—for instance, the Baltic countries, which are on Europe’s front line with Russia.

“In the case of West Berlin in the Cold War, there was no reasonable expectation that a couple of 1,000 U.S. troops could defend against a Soviet attack. But the purpose was to implicate the U.S., to incentivize U.S. involvement if a U.S. soldier was killed by Russia in a Russian attack,” Loss said.

“If you plan to withdraw conventional forces, where is the tripwire that would trigger the threat of U.S. nukes?” he asked. “If there are no U.S. conventional forces on the ground in Europe, how would the U.S. signal commitment to use nukes on allies’ behalf, from the perspective of a nuclear deterrent threat?”

It is not just the overall number but also the lack of diversity in French and British nukes that rattle experts. France and Britain own strategic nukes strictly as deterrents. They are intended to discourage an adversary with the message that no matter the disparity in the total numbers, fewer strategic nukes are far too many to risk a mutually destructive nuclear war.

But the fact that neither the French nor the British have any tactical weapons creates an imbalance.

Loss explained that strategic nuclear weapons have a higher yield and can take out a city. In comparison, tactical nukes have lower yields and can destroy a military command center, industrial areas, and other high-value national assets. But while Russia has tactical nuclear weapons, France and Britain do not.

If Russia uses a tactical nuke on a NATO country, such as those on the front line, Europe is in a pickle without the United States. “How can the Europeans credibly signal that you are willing to use a nuke, too, when they don’t have any tactical weapons?” Loss said.

The answer is in diversifying nuclear capabilities that allow you to respond in a limited manner. “NATO thinks that 100 to 200 weapons are sufficient for this kind of nonstrategic nuclear role,” Loss said. “And that is probably the conversation France and the U.K. would have to have amongst each other but also with interested allies.”

France, unlike Britain, also has an air component and can mount warheads on its Rafale fighter jets in addition to launching them from submarines. It has long advocated strategic autonomy for Europe and was the first to fear a U.S. retreat from Europe after Russia annexed Crimea in 2014. In a speech in 2020 that outlined France’s nuclear policy, Macron called on European partners to engage on “the role played by France’s nuclear deterrence in [European] collective security.”

In a recent paper for the International Institute for Strategic Studies, three scholars highlighted various proposals for a Europeanized French deterrent, either under European Union leadership or with rotating control among European capitals.

Others are more circumspect about normalizing talk around nukes. Chevreuil argues that stronger signaling in the nuclear domain may be sufficient. “Signaling is the first step to show a change in deterrence posture of any nuclear-armed state. It is basically through the most visible, the airborne component. For instance, France has carried out exercises and nuke signaling only in its national territory. But if they were flown over the Baltic nations, it could be seen as France’s commitment and reassure these nations,” she said.

The idea of a nuclear deterrent, experts said, is to never use nukes. If Europe enhances or even talks about enhancing nuclear capabilities, it can encourage nuclear proliferation elsewhere, especially in countries such as South Korea and Saudi Arabia, which face regional adversaries of their own and have contemplated the possibility of obtaining nuclear weapons.