In a major speech this week, Russian President Vladimir Putin announced that he was suspending his country’s participation in the New START treaty, Russia’s only remaining major nuclear arms control agreement with the United States. He also threatened to resume nuclear weapons tests. The declarations sent jitters through the international community. These actions constituted yet another example of Putin’s willingness to leverage his nuclear arsenal, dangling it like the sword of Damocles over the West in order to limit NATO’s support for Ukraine.

Since Russia invaded Ukraine last February, Russian leaders have issued numerous explicit nuclear threats against Ukraine and NATO. In April, Putin promised to respond to outside intervention in the conflict with “swift, lightning fast” retribution. “We have all the tools for this,” he added, “ones that no one can brag about.” So far, however, there has been no significant or observable change in the operational readiness of nuclear weapons in either Russia or in Western countries.

Some observers see Russia’s decision to not use nuclear weapons yet as proof that it will never do so. But that assessment assumes Putin is a rational actor and would not risk the calamity and the pariah status that would follow any Russian deployment of such a weapon. Unfortunately, it is far from clear that Russia’s nuclear brinkmanship is mere bluffing. Moreover, nuclear weapons in the war in Ukraine are not remarkable in their absence, but rather in how they frame the conflict. By deterring the greater intervention of NATO, the Russian nuclear arsenal has helped prolong the war and make any conventional resolution to the fighting more difficult to attain. The conflict in Ukraine is no doubt the most dangerous nuclear confrontation since the 1962 Cuban missile crisis. As the past year of carnage and bluster has shown, nuclear weapons wield devastating power even as they remain locked in their silos—and governments need to reinforce the taboo against their use.

DEADLY DETERRENCE

In the context of the Ukraine war, nuclear weapons have mostly benefited Russia. Putin has invoked his nuclear might to deter NATO from any military intervention on Ukraine’s behalf. That deterrence has worked: the West is (rationally) unwilling to enter the war directly or even to give Ukraine long-range firepower that could reach far into Russia, for fear that such help could end up sparking an apocalyptic nuclear conflict. As a result, the war will likely last longer than it would have if the West entered the fray. A longer war will lead to many more deaths and further destruction. Were nuclear weapons not in the calculus, the United States and NATO would be able to employ their superior conventional firepower more effectively in Ukraine’s defense to win the war quickly. But Putin’s nukes neutralize the West’s conventional military superiority.

It is also possible that Russia’s nuclear weapons emboldened Putin to invade in the first place, because he would not have attacked Ukraine without a way of keeping the United States and NATO out of the war. Of course, Putin acutely misjudged the relative strength of the Russian military. But Russian leaders are aware of their conventional military’s inferiority to that of the West. The fact that Russian leaders issued so many explicit nuclear threats suggests that they saw their nuclear arsenal as a way of compensating.

To be sure, the nuclear weapons in the arsenals of several NATO member states presumably have deterred Russia from expanding the war to NATO countries, such as Poland, Romania, or the Baltic states. In this regard, nuclear deterrence has clearly helped prevent a wider war.

But it has also prolonged the conventional war, at greater cost to everyone, especially the Ukrainian people. A grinding, brutal war of attrition could persist for a long time, with no side able to land a definitive knockout blow. In such a war, Russia maintains a significant advantage over Ukraine by virtue of its much bigger population and larger military.

A PERILOUS MOMENT

Some Western analysts suggest that the United States and NATO should call the Kremlin’s bluff—they should more forthrightly back the Ukrainians and drive Russian forces out of Ukraine. Russian leaders have repeatedly warned of escalation if the West keeps arming Ukraine, but, the argument goes, the Kremlin will not actually resort to nuclear weapons and break the taboo regarding their use. As a result, many observers, mostly outside government, are taking a cavalier approach to the risk of nuclear escalation.

Some pundits take the fact that Putin has not used nuclear weapons after a year of embarrassing military defeats as evidence that he will not use a nuclear weapon in the future. They argue that the West should do whatever it takes to support Ukraine. They criticize U.S. President Joe Biden for declining to send advanced military equipment to Ukraine and deride the supposed defeatists who fret about escalation. “The greatest nuclear threat we face is a Russian victory,” the journalist Eric Schlosser wrote in January in The Atlantic. The historian Timothy Snyder, one of the most perceptive observers of the war, has dismissed Russian threats as mere “talk” designed to scare the West. In February, he went so far as to mock people concerned about nuclear escalation, writing that discussions of the risks of nuclear war are mere media “clickbait” and “a way to claim victimhood” and “blame the actual victims.” But some close observers of Putin, such as the writer Masha Gessen, disagree. They are much less sanguine about Putin’s rationality. In the warped worldview of the Russian president, Gessen has argued, the use of nuclear weapons could be justified as a rational course of action.

Russia’s decision to suspend implementation of the New START agreement—the last remaining treaty limiting the world’s two largest nuclear arsenals—is a deeply disappointing development that increases nuclear dangers. It appears that Russia will no longer participate in the system of mutual on-site inspections and exchanges of information regarding each side’s nuclear stockpile. These information exchanges were crucial confidence-building measures and also comprised one of the last few remaining regularized channels of communication between the United States and Russia about their nuclear arsenals. The Russian Ministry of Foreign Affairs confirmed that Russia will continue to observe limits on the number of nuclear warheads it can deploy under the treaty (1,550 deployed strategic warheads and 700 deployed strategic delivery vehicles). But this suspension makes it more likely that, after New START reaches its scheduled expiration in 2026, it won’t be replaced. Without a replacement treaty, there will be no restraints on the U.S. and Russian nuclear arsenals for the first time since 1972. This is a recipe for a dangerous new arms race.

There is no easy solution to the bind Western governments find themselves in: deterred by the potentially phantom menace of the Russian bomb. Such are the geopolitical consequences of a world with nuclear weapons. Critics of the West’s cautious behavior deride it as “self-deterrence,” but it’s just deterrence, plain and simple. During the Cold War, the West did not respond militarily when the Soviet Union invaded Hungary in 1956 and Czechoslovakia in 1968. Western leaders stayed on the sidelines owing to the unacceptable risk of nuclear escalation.

It’s important for Ukraine to win the war because Russia’s unprovoked aggression challenges fundamental international norms of the territorial integrity of states. But what a Ukrainian victory looks like remains unclear. In a world without nuclear weapons, a military victory would appear fairly straightforward: the recovery of all of Ukraine’s territory, including Crimea. But under the shadow of nuclear weapons, such a victory may not be achievable. A good outcome for Kyiv will be more complicated to attain, and invariably less satisfying.

The challenge Ukraine’s defenders face is how to prevent Russia from benefiting from its nuclear brinkmanship while still avoiding nuclear war. The tendency of wars to expand poses a real risk of escalation. Some commenters have applauded the West for its slow but steady increase of ever more lethal assistance to Ukraine. They frame it as a clever strategy of gradually enhancing Ukraine’s firepower in a way that is not overtly confrontational. In January, for example, the West finally dropped its long-standing opposition to supplying Ukraine with tanks. Kyiv immediately raised its requests for Western fighter planes, which would allow it to strike far into Russian territory.

Neither U.S. nor European leaders seem to have a clear sense of where the redlines are in Ukraine. But they cannot risk finding out too late that their measures of support have incurred the ultimate Russian response. As Kristin Ven Bruusgaard, an expert on Russian nuclear doctrine, wrote in Foreign Affairs in February, a “perilous moment will come if Russian military or political leaders decide that a direct military confrontation with NATO is inevitable.” She warned the West against taking steps that Russian leaders could misinterpret as preparations for a military operation against Russia. Supplying more lethal weapons to Ukraine, offering more intelligence to help the Ukrainians target Russian personnel and military infrastructure, or sending military advisers to Ukraine may not reflect any Western intention to attack Russia. But when such actions are accompanied by talk about taking back Crimea, for instance, or winning a “total victory” for Ukraine, or even “weakening Russia,” it fuels the perception of Russian leaders that a hostile West seeks to destroy their country. However unfounded these views may be, Western leaders have a moral obligation to take seriously the possibility of a catastrophic misunderstanding.

The interests of the United States and Ukraine may overlap, but they are not identical. Ukrainian President Volodymyr Zelensky is doing exactly what one would expect of him: exhorting the West to provide more weapons. But Biden has different priorities and obligations. His job is to make sure that the war does not escalate into a nuclear conflict with Russia. No one wants Russian nuclear blackmail to succeed, for both moral and strategic reasons. But responsible Western leaders still have to weigh seriously the probability of a calamitous event. That means there will need to be continued and significant limits on Western assistance to Ukraine.

THE UNBROKEN TABOO

The course of the war will also shape the fate of a fundamental international norm: the taboo against the first use of nuclear weapons. Russia’s nuclear threats have been harmful for the nuclear taboo because they suggest that the use of these massively destructive weapons is legitimate and a plausible part of war. Yet the taboo continues to hold and, so far, restrain Russia and NATO.

The world’s response to Russia’s nuclear threats has played a crucial role in reinforcing the taboo. World leaders from all continents, including Russia’s friends in China and India, have made clear to Putin that nuclear use would be unacceptable. Chinese President Xi Jinping said in early November that the world should “jointly oppose the use of, or threats to use, nuclear weapons.” He later added in meetings with Biden that nuclear use in Ukraine was “totally unacceptable.” The UN secretary-general and diplomats at many UN meetings have condemned Russia’s nuclear threats in their speeches and statements. Government officials, analysts, and journalists have explicitly mentioned the taboo and noted the importance of upholding it. It has undoubtedly become clearer to Putin that violating the taboo would likely alienate countries that have either supported the Kremlin or remained neutral: a Russia that breaks the taboo would instantly become a pariah.

Western policymakers have sought to defuse the possibility of nuclear conflict at every turn. The United States has not responded to Russia’s nuclear bluster with either threats of its own or any change in the posture of its nuclear forces. Instead, if Russia were to launch a nuclear war, U.S. officials have promised an overwhelming conventional military response, not retaliation in kind. This is exactly the right approach to upholding the taboo. If the goal is to isolate the norm violator, it is important to avoid violating those norms, as well.

Still, the risk remains that Russia will use a nuclear weapon in Ukraine. A troubling new development is the nuclear militancy expressed in Russian society, especially on Russia’s state-controlled television, where hosts regularly urge the use of nuclear weapons against the West. “Russia’s popular culture is now marked by a level of nuclear fanaticism previously associated with North Korea,” Schlosser wrote. “Nothing like it existed during the Cold War.” Dictators may be relatively more shielded from the pressures of public opinion than leaders of democracies, but nuclear fanaticism anywhere threatens the taboo.

It is impossible to say definitively whether greater Western support for Ukraine will prompt a nuclear Russian response. No one really knows. The nuclear risks in this war are considerable, since NATO continues to get more deeply involved in Ukraine’s defense while Russia seems less and less restrained (as Putin’s announcement suspending Russia’s participation in New START shows). Deterrence could fail in multiple ways, either through intentional acts or miscalculations. The Russian use of a nuclear weapon would be widely regarded as a failure of U.S. policy. Responsible U.S. leaders will err on the side of caution to avoid such a catastrophic outcome.

The past year saw the continuation of the 77-year tradition of nuclear weapons not being used. Western leaders must do as much as they can to ensure that this streak continues, even as the horrific war in Ukraine rages on.