As the Russo-Ukrainian War grinds on, both Russia and Ukraine have adjusted their strategic objectives. Russia abandoned its initial goal of seizing Kyiv and installing a pro-Russian government after facing fierce Ukrainian resistance, and is now focusing on conquering Eastern Ukraine and annexing significant portions of Ukraine’s southern territory. Ukraine’s minimum objectives include reestablishing its prewar borders, with political leaders occasionally suggesting that Ukraine should expand its ambitions to reclaiming territory lost to Russia in Crimea and the Donbas region since 2014.

U.S. strategic objectives in Ukraine are also a moving target. Secretary of Defense Lloyd Austin stated that the United States not only wants Ukraine to remain a sovereign and democratic country, but also “to see Russia weakened to the degree that it can’t do the kinds of things that it has done in invading Ukraine.” Speaker of the House Nancy Pelosi promised that the United States would support Ukraine “until the fight is done.” President Joe Biden reiterated this point, stating that the United States would support Ukraine for “as long as it takes so Russia cannot, in fact, defeat Ukraine and move beyond Ukraine.”

Analysts and commentators debate how ambitious U.S. support for Ukraine should be. Some scholars have emphasized differences in U.S. and Ukrainian interests and encouraged more limited objectives. Calls for continued and expanded military support have come to dominate the defense policy discourse.

A core point of disagreement between these two camps is the perceived likelihood of nuclear escalation. Whereas those arguing for limited objectives tend to worry about the potential for escalation across the nuclear threshold, analysts in favor of increased support for Ukraine view the costs of concessions as more dangerous than confrontation and tend to view the likelihood of escalation as minimal.

The likelihood of nuclear use in Ukraine may be low, but it is not zero. Analysts who quickly dismiss the potential for nuclear escalation—and even most of those who express worries about nuclear conflict—largely oversimplify the many pathways that can lead to nuclear use, whether purposeful or inadvertent. Providing a clearer delineation of those pathways will help policymakers better understand which policy options can more safely advance U.S. objectives, and which policies should inspire greater caution and restraint.

Choosing to Go Nuclear

On the day that Russia invaded Ukraine, Russian president Vladimir Putin warned that any opposition to Russian efforts would result in consequences “such as you have never seen in your entire history.” Several days later, Putin placed Russian nuclear forces on high alert. Nuclear threats have continued to emanate from Russia throughout the conflict, with Russian foreign minister Sergei Lavrov recently warning Western leaders that the risks of nuclear war are now “considerable.”

Despite such overt nuclear threats, U.S. and European leaders have expressed skepticism about the likelihood of a nuclear exchange. For example, shortly after Russia’s decision to place its nuclear forces on high alert, U.S. president Joe Biden received a question regarding whether U.S. citizens should fear nuclear war in Europe. Biden’s response was simple: “no.” Five months of unfulfilled nuclear threats have ultimately led analysts to argue that Russia’s threats are “not credible.”

Furthermore, given the massively destructive effects of nuclear weapons and apparent taboo against their use, others decisively assert that Russia will not use nuclear weapons in Ukraine because “extraordinary retaliation and universal opprobrium would follow.” Even those observers most concerned about the possibility of Russia using nuclear weapons generally view such an outcome as unlikely.

Such assertions that Russia would not use nuclear weapons rely on an important assumption: the decision to use nuclear weapons will be politically calculated and purposefully directed by Putin. This assumption, however, overlooks a separate challenge to crisis stability. Specifically, crises entail risks of unintended nuclear escalation that occurs without explicit political intent. These concerns merit greater attention when debating U.S. foreign policy with respect to Ukraine, as the likelihood of unintended escalation may be higher than purposeful escalation under certain circumstances, such as if Russia begins mobilizing its nuclear forces to signal resolve.

Pathways to Unintended Escalation

Putin is most likely to consider nuclear weapons use if facing a devastating strategic defeat or existential threat to his regime. Two core interests might fall into this category: first, threats to the physical security of Russia, potentially including challenges to territorial gains made by Russia since 2014; and second, threats to the survival of Putin’s political regime.

Skeptics of escalation concerns argue that nuclear weapons will not come into play as long as the United States and NATO avoid Russia’s red lines, including direct attacks on Russian forces and the deployment of NATO forces into Ukrainian territory. The dangers associated with crossing these red lines explain why U.S. policymakers rejected proposals for a no-fly zone over Ukraine, which would have required Western forces to directly target the Russian military to enforce the policy.

Avoiding direct engagement with Russian forces, however, is insufficient to guarantee that nuclear weapons will not be used. Nuclear use is not a simple on-off switch, and the process of preparing nuclear weapons for potential use entails risks of unintended nuclear use. Simply approaching Russia’s red lines—even without crossing them—increases the likelihood of nuclear use.

The primary node of concern for unintended nuclear escalation during militarized crises is a state’s nuclear command and control systems. Command and control systems are the operational means by which a state conducts the management, deployment, and potential release of nuclear weapons. More simply, command and control procedures determine how centralized a country’s political oversight of nuclear forces is. These systems dictate how a state operates during peacetime and crises, directly shaping the likelihood of nuclear use.

If Putin feels that Russia’s physical security or his political regime are in danger, he is more likely to increase the readiness of his nuclear arsenal. Operationally, this means that lower-level military commanders will be more capable of using nuclear weapons as military operators gain possession of fully prepared and deliverable nuclear weapons, likely without technical controls to inhibit their use. This delegation of nuclear use capability to lower-level commanders creates two risks that have largely been overlooked in the debate about nuclear use in Ukraine.

First, accidental use—referring to the unintentional use of nuclear weapons due to mishandling or poor design—becomes more likely as military operators gain control of fully ready nuclear weapons. Without peacetime barriers to using nuclear weapons—such as the separation of nuclear warheads from ballistic missiles—military operators in possession of nuclear weapons have fewer constraints on their ability to use nuclear weapons. History is replete with examples of nuclear near-misses, instances in which accidents almost resulted in nuclear use, and these events remain entirely plausible in Russia. Moreover, if a nuclear weapon were to detonate accidentally in Ukraine, outside actors likely would not recognize the detonation as accidental and might authorize nuclear retaliation.

Second, the delegation of nuclear use capability would increase the likelihood of unauthorized use, which occurs when the custodians of nuclear weapons use those nuclear weapons without authorization from political leadership. Unauthorized use could occur because a lower-level commander decides to circumvent the chain of command and use a nuclear weapon without political authorization, or that commander may elect to use nuclear weapons to stave off defeat if being conventionally overrun by the adversary. These pressures would be especially pronounced for commanders of Russia’s tactical nuclear weapons, as they are most likely to be placed in a battlefield setting and face “use them or lose them” pressures.

Nuclear Command and Control and Crisis Escalation

During peacetime, Russia appears to manage its nuclear forces in a way that mitigates the risk of accidental and unauthorized use. The Russian president has the centralized ability to authorize the use of nuclear weapons and nuclear warheads are kept de-mated from ballistic missiles, which physically prevents lower-level commanders from possessing nuclear weapons, much less using them.

If an adversary such as NATO approaches Russia’s red lines and threatens the state’s security or Putin’s regime, however, it is possible that Putin would authorize the transfer of nuclear warheads to military operators to increase arsenal readiness in an effort to deter NATO from crossing the red line at all. Placing fully assembled tactical nuclear weapons in the hands of Russian military forces would immediately increase the likelihood of nuclear use by opening doors to accidental or unauthorized use.

Beyond accidental and unauthorized use, this process of increasing arsenal readiness entails another significant threat to strategic stability. The unusual release of sensitive information by the Biden administration shows that the United States is actively monitoring every available aspect of the conflict in Ukraine. If U.S. intelligence were to discover that Russia was removing nuclear warheads from storage and increasing operational readiness, U.S. policymakers will be forced to make decisions about supporting military efforts that are apparently approaching the nuclear brink without knowing whether Russia was simply increasing arsenal readiness or actually preparing to conduct a nuclear strike. In this case, Western officials could view Russian mobilization as a cause for preemptive strikes against Russia’s tactical nuclear weapons. Whether NATO conducts the attack or passes intelligence to Ukraine in support of an attack, this direct targeting of Russian nuclear forces would clearly cross a red line that encourages nuclear use, thereby guaranteeing the nuclear exchange that attacking forces hoped to avoid in the first place.