Deterring Nuclear Weapons Use in Ukraine

Nuclear weapons have been lurking in the background since the start of the Ukraine crisis on February 24. President Vladimir Putin has increasingly relied on nuclear threats in an attempt to prevent foreign intervention and signal to the United States and NATO that he is committed to winning the war in Ukraine. For example, on September 21, he stated, “Our country has different types of weapons as well, and some of them are more modern than the weapons NATO countries have. In the event of a threat to the territorial integrity of our country and to defend Russia and our people, we will certainly make use of all weapon systems available to us. This is not a bluff. . . . Those who are using nuclear blackmail against us should know that the wind rose can turn around.” To deter Putin from using nuclear weapons in Ukraine, the Biden administration has been forced to find a delicate balance of avoiding overt threats accompanied with private deterrence signals. One of the main reasons for this is to maintain NATO unity; some allies would prefer to see explicit threats against nuclear weapons use, and others would view such threats as escalatory and dangerous.

Q1: What is the role of nuclear weapons in the war in Ukraine?

A1: Putin has referred to nuclear weapons on at least three occasions since the invasion of Ukraine. On February 27, he ordered Russia’s nuclear forces be put on “special combat readiness,” leading some experts and news outlets to interpret this as “high alert” nuclear status, although in practice there were no noticeable changes to Russia’s nuclear forces. The second occasion was his September 21 speech, given in response to perceived nuclear threats from NATO. In the same speech, Putin announced the partial mobilization of up to 300,000 reservists, indicating Russia was losing ground in Ukraine and is being increasingly drawn into a quagmire. More recently in an October 3 address, Putin argued that the United States was the only country to have used nuclear weapons in war. “By the way, they created a precedent,” he ventured in an aside. This weak analogy neglects nearly 77 years of the nuclear taboo.

The United States and NATO have been noticeably restrained in mentioning nuclear weapons or making any nuclear threats. NATO’s nuclear doctrine, like the United States’, relies on strategic ambiguity and says, “The fundamental purpose of NATO’s nuclear capability is to preserve peace, prevent coercion and deter aggression.” Thus far, NATO has been successful at deterring Russia from directly attacking a NATO member and from using weapons of mass destruction in Ukraine. Maintaining that deterrence and alliance unity should be a priority for NATO going forward.

Q2: Why might Putin use nuclear weapons in Ukraine?

A2: As Russia suffered battlefield losses and defeats, Putin’s nuclear saber-rattling is an effort to convince NATO he would use nuclear weapons in Ukraine to deter NATO from directly intervening in the crisis or escalating the war. To be clear, Putin wants the West to think he would use nuclear weapons. That does not necessarily mean he would, but it does not mean such threats should be taken lightly. Russia’s nuclear strategy can be described as one of “escalation management, seeking to dissuade, intimidate, or achieve de-escalation at key transition points and early phases of conflict, from peacetime through large-scale and nuclear war.” Russia would use nuclear weapons in the face of existential threats or if the survival of the state was at risk. While this might seem a far cry from the current situation in Ukraine, if Russia absorbs parts of Ukraine through undemocratic elections or if Ukraine threatens to seize Russian territory, this could be perceived as grounds for escalation in the Kremlin. Prior to such threats, however, Putin might turn to nuclear weapons to demonstrate resolve and a commitment to winning the war in Ukraine.

Q3: How can the United States and NATO allies deter nuclear weapons use in Ukraine?

A3: Deterrence entails threatening to impose costs if an adversary takes an unwanted, aggressive action. Since deterrence is ultimately about perception, it needs to be tailored to the adversary. Deterring Putin therefore will require either threatening to retaliate and punish him or refusing to afford him the desired outcome of any nuclear use, i.e., winning the war in Ukraine or gaining regional preeminence. These costs could be military in nature, such as U.S. or NATO conventional intervention in Ukraine, or the costs could be economic or reputational, such as turning Russia into a pariah state similar to North Korea. Perhaps one of the biggest costs to Putin if he used nuclear weapons could be domestic backlash. As Lawrence Freedman has observed, “It is also hard to imagine that [Russian use of nuclear weapons] would be greeted calmly in Russia. It could intensify opposition in Moscow to Putin.” Additionally, the Biden administration and NATO can signal to Putin that nuclear use will not win him any advantages or achieve his ultimate objective of preventing Ukraine and other countries on Russia’s borders from turning toward the West.

But successfully deterring Putin will also require maintaining NATO unity. One of the greatest weapons against Russia’s invasion, aside from the Ukrainian people, has proven to be the alliance’s unity and continued support for Ukraine in the face of unprovoked aggression. If U.S. efforts to deter Putin are seen as escalatory or dangerous by some NATO allies and undermines unity, that would work at cross-purposes and could embolden Putin.   

Q4: What has the Biden administration done so far to deter nuclear weapons use in Ukraine?

A4: The Biden administration and NATO leadership have had to strike a delicate balance in deterring nuclear use in Ukraine. Both sides have refrained from explicitly threatening a military response to any Russian nuclear weapons use. Instead, the Biden administration has taken a three-pronged approach. First, the president has been clear that Putin’s threats should not be taken lightly, and the threat of nuclear use is a serious matter. On October 3, Biden stated, “He’s not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons because his military is, you might say, significantly underperforming.” Second, the administration has avoided any public mention of a military or nuclear response to a Russian nuclear attack. Instead, the U.S.  president has used ambiguous and careful language, such as “You will change the face of war unlike anything since World War II.” Finally, the administration has been sending private messages to Moscow about the “grave consequences” it would face in the event of a nuclear weapons use in Ukraine. This public-private messaging is not new but is particularly important in the current crisis not only to avoid escalation but also to assure NATO allies.

Q5: What would happen if nuclear weapons were used in Ukraine?

A5: The war in Ukraine has already led to mass human displacement, with over 7 million Ukrainians now living outside Ukraine, along with major disruption to energy and food supplies and other potentially dangerous “ripple effects.” The humanitarian, ecological, and economic consequences of nuclear weapons use would be orders of magnitude greater and could cross borders to also affect Belarus and parts of Europe. The impacts of nuclear use would depend on a variety of factors, including the size of the nuclear weapon used and the location where it was used. A relatively low-yield nuclear explosion over the Black Sea, for example, may result in a relatively small number of casualties, aside from environmental affects. A small tactical nuclear weapon detonated at surface-level in eastern Ukraine might have low casualties but could create radioactive debris that would blow into surrounding areas, including potentially into Russia.

The United States and NATO would not need to respond in-kind to a Russian nuclear use. Any nuclear use, no matter how small, would break a 77-year taboo and could leave Russia an international pariah, abandoned by partners such as China and India. Therefore, while the U.S. military response to nuclear weapons use might depend on the number of casualties, as recently argued by former senator Sam Nunn, any nuclear use warrants a blanket response of international opprobrium. If the United States and NATO did decide to intervene, either because of direct threats to a NATO member or simply to put an end to Putin’s aggression, the allies have sufficient conventional options to mount a military response that would avoid further nuclear use and escalation. The goal, at present, should remain deterring Putin from further escalating the war and maintaining NATO unity. 

Heather Williams is director of the Project on Nuclear Issues and senior fellow with the International Security Program at the Center for Strategic and International Studies in Washington, D.C.

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Director, Project on Nuclear Issues and Senior Fellow, International Security Program