OPTIONS IF RUSSIA GOES NUCLEAR

Today, Russia possesses two thousand tactical nuclear weapons that it could deploy by plane, missile, or ship. It would most likely use the short-range Iskander-M missile system. These weapons have yields of 1–50 kilotons, the largest of which would have a blast radius about half a kilometer wider than the bomb the U.S. military dropped on Hiroshima, Japan, at the end of World War II.

Russia’s use of a tactical nuclear weapon against military targets on the battlefield would be unlikely to halt the Ukrainian counteroffensive. Even if a battlefield nuclear weapon made a land advance more challenging, Ukraine would likely continue its focus on aerial attacks and air defense in ways that have been successful in recent weeks.

The environmental effects of tactical nuclear weapons use are difficult to calculate and would depend on warhead yield, detonation height, weather, and local geography. Russia would be cautious not to detonate weapons too close to its own soldiers or occupied territory.

China would almost certainly be forced to publicly denounce such weapons use by Russia, and the resulting distancing would have repercussions for future China-Russia cooperation more broadly.

Options

1. Putin may well use a low-yield tactical nuclear weapon against a Ukrainian military target in order to minimize the scale of civilian casualties (and to discourage a US/NATO response in kind). He may hope that just by crossing the nuclear threshold, he would so shock Ukraine and its Western backers—and so terrify allied publics—that they would back down rather than risk further escalation. The sequence would look like this: Putin would first create a “provocation,” then hit a high-value target such as Kyiv in hopes of getting Ukraine to accept “peace at any cost.” However, he would also seek to avoid any potential spillover (such as radiation) into any NATO state.
2. If Putin uses nuclear weapons, the United States and its allies would face only grim options. Launching a disarming conventional or nuclear attack in response would be insane, since it would risk massive retaliation (and perhaps the end of civilization). Engaging in an escalatory tit-for-tat would be foolish because Putin has much more at stake—his regime’s survival, and perhaps even his life—than the West does. Tightening sanctions would merely increase the risk to Putin’s regime, which would fuel incentives for further nuclear escalation. The only wise response to Putin’s nuclear use in Ukraine would be to negotiate some kind of resolution in which all parties could declare Potemkin victories. If that is the chosen path, it makes far more sense now to dial down US rhetoric about regime change or decisive victory and, instead, find a solution before nuclear weapons are used. 
3. Assuming the United States, NATO, and Ukraine could agree on any action, there are several possibilities. If the Russian attack caused little damage, NATO might first try to issue an ultimatum with the aim of reaching a settlement on the Alliance’s and Ukraine’s terms. But if Putin had convinced himself that there was no other option but to strike, he would be unlikely to acquiesce to any allied proposals for such a settlement. Similarly, a non-nuclear military response (for example, conventional strikes on military bases and infrastructure in Russian territory that are supporting the invasion) would probably not be decisive and would appear inadequate to many—in addition to carrying its own risks of escalation. That leaves the option, for which NATO forces are fully adequate, of a strike tailored to the scale and character of the Russian one. Some would understandably argue that continuing to defend Ukraine is simply not worth risking a nuclear escalation. But while it is easy to see the risks of a nuclear counterstrike, there are also serious implications of not doing it: The absence of a US nuclear response would gravely weaken the credibility among both friends and adversaries of the entire strategy of deterring nuclear attack through the prospect of US nuclear retaliation. This, in turn, would make a bigger war more likely. May we never need to face the choice.
4. If Putin resorted to the use of nuclear weapons, the United States and its allies would need to respond quickly and decisively to ensure that Putin paid a heavy price for crossing the nuclear threshold. An initial response, calibrated to reduce the risks of escalation, could involve a major strike with conventional weapons against high-value Russian military targets involved in the war against Ukraine (such as the Black Sea Fleet). But the United States and its allies should maintain ambiguity in their declaratory policy as to whether they would respond in kind, rather than using only conventional forces. Such ambiguity regarding the nature and scope of the response would be more effective in deterring Putin from using nuclear weapons in the first place. Allies should consult now on possible response options so that they are not paralyzed with indecision when crunch time comes. 
5. The United States and its allies should deploy conventional weapons to quickly defeat Putin’s military in Ukraine and make clear that his use of nuclear weapons achieved nothing but defeat. In addition to providing an even higher level of military support, Ukraine’s Western partners would deploy naval forces to the Black Sea to destroy all Russian ships as well as Russia’s bridge to Crimea (a thirty-day mission); give Ukraine air superiority using Patriot missiles and, if necessary, US advanced jet fighters; and station US and allied intelligence, logistics, and humanitarian support units inside Ukraine.