On February 24, as he launched the invasion of Ukraine, Russian President Vladimir Putin raised the specter of nuclear war. He warned that any outside interference would lead to consequences “never seen in history,” a clear reference to the use of atomic weapons. The prospect of an outright nuclear war with Russia may indeed have convinced NATO member states that their direct involvement in the conflict was too dangerous to countenance. But the war did not follow Putin’s script. Russia’s surprising early struggles on the battlefield, along with Ukraine’s dogged resistance, persuaded many NATO members to send military equipment and supplies to aid the defenders.

Putin again invoked nuclear war in April. After his forces retreated from the outskirts of Kyiv and Kharkiv, he declared that Russia would employ nuclear weapons “if necessary” to achieve its aims. With such rhetoric, Putin may have been trying to prevent NATO from boosting its support for Ukraine to levels that would achieve an outright defeat of the Russian invasion. If so, Putin succeeded, and a rough stalemate between the belligerents ensued. Putin’s nuclear saber rattling quieted during the summer as both sides sought to gain an advantage on the battlefield. In early autumn, having failed to change the military balance in his favor, Putin returned to nuclear threats, declaring that “to defend Russia and our people, we will certainly make use of all weapon systems available to us. This is not a bluff.”

Many Western leaders think that Putin’s recurring cries of “nuclear wolf” mean he is bluffing. His tough talk seeks to sow doubt and fear in the minds of his adversaries, they suggest, but he would never actually detonate a bomb. In other words, they insist that Putin is too rational to risk the potential catastrophe of nuclear war. But that is an assumption the West cannot afford to make. Driven by his desire to restore an injured Russia to greatness, unshakable in the righteousness of his claims to Ukraine, inspired by earlier successes in Crimea and the Donbas in 2014, and increasingly desperate in the face of Russian military failure and international hostility, the Russian president could indeed see the virtue of resorting to nuclear weapons. Although the odds of Putin’s crossing the nuclear threshold may be low, prudence dictates that NATO cannot discount his taking such a dangerous course of action. Western leaders must determine how they can prevent such an escalation from spiraling into utter catastrophe.

WITHIN REASON

U.S. President Joe Biden is taking the Kremlin’s pronouncements seriously, asserting in October that for the first time since the 1962 Cuban missile crisis, there is now “a direct threat” of nuclear war. Putin, according to the U.S. president, is “not joking.” Yet Biden also sees Putin as a “rational actor who has miscalculated significantly” and who will avoid using nuclear weapons. Biden’s tempered assessment of Putin has been seconded by others, including the United Kingdom’s defense secretary, Ben Wallace, who said in October that Putin was “highly unlikely” to resort to nuclear weapons and that the Russian president recognized that restraint was in his best interest.

The notion that rational decision-makers will not risk nuclear war has been a common refrain since the dawn of the nuclear age. In 1954, when discussing the possibility of a preemptive strike against China, U.S. President Dwight Eisenhower told the joint chiefs that in a nuclear war, “there is no victory except through our imaginations.” Hubert Vedrine, who served as France’s foreign minister from 1997 to 2002, echoed that view when discussing the possibility of Iran acquiring atomic weapons. He told The New York Times in 2007 that possession of nuclear weapons was in fact a guarantee of not using them: “A country that possesses the bomb does not use it and automatically enters the system of deterrence and doesn’t take absurd risks.” Vedrine was in effect suggesting that only a madman would risk triggering a nuclear war. He presumed that humans are rational beings, in the sense that they act to maximize the prospective benefits of their actions, and they are constantly weighing the associated costs, benefits, and risks of their decisions.

But human beings may not be such calculating creatures. There is considerable evidence, both from history and from recent advances in cognitive science, suggesting that humans do not always act rationally. Research reveals, for instance, that all other factors being equal, people are generally less willing to take risks to acquire what they do not have than to preserve something of equal value that is already theirs—“prospect theory,” as the phenomenon is known in behavioral economics. This tendency appears to be good news for those seeking to deter the aggressive behavior of others, since people are relatively risk averse when it comes to pursuing what is not theirs.

But there’s a catch. Rival actors may both believe that they are vying over something that is rightfully theirs. Such is the case with Ukraine. Putin views it as a wayward province of Russia that must be recovered. Ukrainians and their NATO backers see it as an independent country that must be defended from the Kremlin’s depredations. Thus both sides are willing to engage in relatively high-risk behavior to possess what they believe is theirs.

Leaders can also dangerously overrate their position owing to “optimism bias.” Research in the cognitive sciences finds that political leaders have an inflated confidence in their ability to control events, making them more willing to take on risks. That misguided self-belief makes them more prone to double down when facing failure, taking greater risks rather than cutting their losses.

Despots are particularly prone to optimism bias. Like other politicians, they have beaten the odds to reach their lofty perches. But unlike political leaders in democracies, a prospective dictator who fails to seize power does not end up in the loyal opposition but often faces imprisonment or even death. Successful tyrants are, therefore, individuals who have an unusually high tolerance for taking risks. This appears to be the case with Putin. Instead of looking for an off-ramp out of the war, he has escalated the conflict, undertaking a partial mobilization of Russian manpower and annexing four Ukrainian oblasts while widening his attacks to include missile strikes on Ukraine’s critical energy infrastructure. These actions conform to prospect theory’s finding that Putin will take greater risks to secure what he believes is rightfully his.

Leaders make some decisions that appear irrational when viewed from a strictly transactional perspective but less so when taking into account perceptions of honor and justice. In Putin’s case, he seems convinced that his war is a mission to restore Russia’s national honor, redeeming the country from the humiliation that followed the end of the Cold War and the disintegration of the Soviet Union. In his annual state of the nation address to parliament in 2005, he referred to the Soviet Union’s collapse as “the greatest geopolitical catastrophe of the century” and viewed as unacceptable that “tens of millions of our fellow citizens and countrymen found themselves beyond the fringes of Russian territory.” For Putin, Russia’s honor requires the recovery of these “lost” Russians and the land they occupy, even if he must run high risks to achieve this.

History is rife with examples of when matters of honor and justice made leaders take actions that were not, on their face, rational. For example, during the Cuban missile crisis, Soviet leader Nikita Khrushchev was willing to risk nuclear war over the issue of U.S. nuclear-armed missiles in Turkey, which constituted a trivial portion of Washington’s overall nuclear arsenal. Khrushchev’s decisions appear irrational—until one accounts for his belief that it was unfair for the Americans to house nuclear missiles on Russia’s border while demanding the withdrawal of Soviet missiles from Cuba.

Putin may reach a point where he believes he has nothing left to lose. Perhaps the coming months will find the war moving inexorably against him. Europe may muddle through a cold winter without caving in to Putin’s pressure. Russia’s growing economic distress could trigger internal unrest. Spring might bring renewed Ukrainian offensives that gradually chip away at Russia’s control of eastern Ukraine and perhaps even liberate Crimea. Large-scale desertions among Russian conscripts might follow. At that point, the restoration of Ukraine’s 1991 borders (those that obtained before Putin’s 2014 incursion and annexation of Crimea) would seem increasingly likely. As the war’s architect, Putin would find it difficult to deflect blame from himself. A war that was supposed to end in swift victory for Russia would have backfired calamitously. In such circumstances, Putin might be willing to run extraordinary risks, including the use of nuclear weapons—a cosmic roll of the dice—in attempting to avoid a humiliating defeat. As Winston Churchill put it in a 1955 speech to the House of Commons, “the deterrent does not cover the case of lunatics or dictators in the mood of Hitler when he found himself in his final dug-out.”

RESPONSES TO MADNESS

Given these considerations, the West cannot discount the possibility that Putin will turn to nuclear weapons to compel Ukraine and NATO to seek peace on terms favorable to him. There are myriad scenarios for how he may choose to cross the nuclear threshold. Putin could take a shot across the bow of his foes—detonating a single nuclear weapon to demonstrate Russia’s willingness to escalate to widespread nuclear weapons use if his demands are not met. A strike of this kind could take a number of forms, including an airburst (the explosion, in such a case, of a nuclear device in the air) that does little if any damage, or perhaps an attack on Ukrainian vessels in the Black Sea.

The Kremlin might target one of Ukraine’s nuclear reactors with a conventional strike, creating a modern-day environmental “Chernobyl” with radioactive material spewing into the atmosphere and the surrounding environs. Without actually using nuclear weapons, Moscow could still precipitate nuclear weapon effects. The message, however, would be the same and uncompromising: Russia’s enemies need to back down and find a way out of the war before things get even worse.

Putin would succeed only if either of these gambits persuaded panicking Western governments to pressure Ukraine into a modern-day Munich Agreement (the 1938 deal that attempted to appease Nazi Germany by granting it a swath of Czechoslovakia), making territorial concessions before the Kremlin escalated further. Putin could perhaps end up pocketing his annexations in the east and south of Ukraine. Such a coup for the Kremlin would deal a devastating blow to the fundaments of nuclear deterrence and the taboo against using nuclear weapons. Putin would have shown that nuclear attacks can achieve political aims, lowering the bar for their future use. Following Putin’s lead, one can imagine Chinese President Xi Jinping envisioning a nuclear demonstration to coerce Taiwan and its prospective defenders.

But if these opening salvos—a nuclear warning shot or the destruction of a nuclear plant—failed to cow Ukraine and the West, Putin could take a further step: using nuclear weapons on the battlefield. Russia has retained several thousand tactical nuclear weapons, which typically have far smaller yield, or destructive power, than those of its strategic nuclear weapons positioned on long-range delivery systems capable of reaching the United States. For example, consider a five-kiloton nuclear weapon that has roughly 40 percent of the yield of the bomb dropped on Hiroshima. This weapon’s effective kill radius extends for roughly only a mile, with even a shorter radius for troops that have taken shelter, such as inside their tanks. Consequently, creating a major breach in the Ukrainian defenses would likely require employing dozens of these weapons. Of course, Russian troops would have to advance through this radiation-contaminated breach, which presumes a level of courage and combat proficiency they have yet to evince in this war.

Yet even if such strikes managed to inflict high casualties on personnel and destroy Ukrainian equipment, it is not clear that the military balance would shift decisively in Putin’s favor. The United States could respond by engaging in operations solely over Ukrainian territory. As the U.S. military has shown again and again, conventional air power can stop ground forces in their tracks. Such was the case during the North Vietnamese Easter Offensive in 1972, the air campaign in Iraq and Kuwait in 1991, and in the skies over Afghanistan in 2001.

Finally, Putin may choose to use nuclear weapons in the way they are most feared: as an instrument of terror, such as in a strike on a city. Some Russian tactical nuclear weapon yields exceed 100 kilotons, nearly ten times that of the Hiroshima bomb. Were such a weapon used against Kyiv, casualties could exceed 100,000. Although Putin might hope this would shock his enemies into seeking peace, it would also risk triggering Armageddon. In seeking to restore Russia’s greatness, Putin would risk his country’s destruction.

THINKING THE UNTHINKABLE

The period of U.S. dominance that followed the Cold War lulled the world into complacency regarding nuclear weapons. The return of great-power politics and revanchist great powers such as Russia has stripped these illusions away, and the West would be foolish to rule out the possibility of Putin approving nuclear strikes. History and the growing understanding of how the human mind works suggests that leaders should not assume that deterrence will be robust and the taboo against atomic weapons will prevent their use. This is especially the case when dealing with a leader such as Putin who seems convinced of the justice of his cause, who fights for the supposed honor of his country, and who is becoming desperate.

It is equally important, however, to understand that, even in the wake of a limited Russian nuclear attack, the West has plausible options for continuing to support Ukraine’s self-defense and demonstrating that Russia’s use of nuclear weapons will lead to failure, rather than success. Under such conditions, it may be possible to stuff the nuclear genie back in his bottle.

Western leaders must try to make Putin realize, as he considers turning to his nuclear arsenal, that there can be no winners in such a conflict. He should understand that any use of nuclear weapons would place Russia and the West on a slippery slope, moving down a path that neither truly desires.