Why the War in Ukraine Won’t Spark a Nuclear Proliferation Cascade

Soon after Russian President Vladimir Putin launched his renewed invasion of Ukraine in February, analysts began warning of dire consequences for the global nuclear order.

Russia’s invasion, the argument goes, shows that nuclear weapons can provide a shield that enables conquest. After all, Putin has used nuclear threats from the outset of the war to help deter Western intervention, and U.S. President Joe Biden himself has made clear the prospect of a nuclear confrontation with Russia, powerfully shaping his caution about direct U.S. involvement.

At the same time, Russian aggression shows how difficult it is for nonnuclear states to deter their nuclear adversaries. According to this line of argument, if only Ukraine had its own nuclear arsenal, Russia wouldn’t have dared attack it—just like the United States probably wouldn’t have attacked a nuclear-armed Iraq under former President Saddam Hussein or a nuclear-armed Libya under former leader Muammar al-Qaddafi. (Ukraine relinquished the Soviet nuclear weapons, which it never had operational control over, that were left on its territory in the 1990s after the Soviet Union’s collapse.)

The lesson thus seems clear: Whether you’re a status quo power or a dissatisfied aggressor, you’ll be better off with the bomb. According to Yale University historian Timothy Snyder, who has taken the argument perhaps the farthest, unless Ukraine dispels these lessons by defeating Russia, the result will be “global nuclear proliferation.”

Commentators are often not specific about exactly which countries they believe will take part in this cascade, but one can imagine Iran concluding a nuclear arsenal would give it more leeway in the Middle East, which would likely then lead Saudi Arabia to pursue a bomb program. Likewise, Taiwan or South Korea might become increasingly worried about facing an invasion by their nuclear-armed adversaries, causing them to seek a nuclear arsenal.

Although the logic of these arguments is persuasive at first glance, they are not well supported by the history of nuclear proliferation, in large part because they overlook the complexity of nuclear decision-making.

From the 1950s onward, analysts and government officials have been predicting proliferation cascades or domino effects, where there would be a rapid increase in the number of states with nuclear weapons. Most famously, then-U.S. President John F. Kennedy warned in 1963 that he foresaw the possibility of as many as 25 nuclear-armed states by 1980 unless the international community got a handle on the problem.

These predictions were never borne out: The number of states with nuclear weapons has grown rather slowly, especially since the 1970s. Between 1945 and 1970, six states acquired the bomb (about one every four years). Since then, only four countries have additionally done so (about one every 13 years).

Why has proliferation proceeded so slowly? In large part because leaders like Kennedy worried about it and adapted their policies to prevent proliferation cascades. Especially since the 1960s, a powerful architecture for preventing the spread of nuclear weapons has been constructed. This includes the Non-Proliferation Treaty, economic sanctions policies adopted by the United States and others, the Nuclear Suppliers Group, and sometimes the threat and use of force—all of which have been shown to have restrained the spread of nuclear weapons.

This architecture is still in place today and in some ways stronger than it has ever been. Today, almost every state without nuclear weapons is a member of the Non-Proliferation Treaty, and perhaps for the first time since 1945, there are no states currently known to be seeking nuclear weapons. (Iran is producing enriched uranium very close to weapons grade, but according to the U.S. intelligence community, as of December 2021, there is no evidence it has made the decision to build a bomb.)

We also know from recent research that proliferation outcomes are not determined by security incentives in the mechanistic way predicted by those worried about the war in Ukraine’s impact. In other words, states that you may think would want nuclear weapons for protection or aggression often do not try to get them, and states that try to get them sometimes fail due to domestic pathologies or divisions.

Many states in threatening security environments are satisfied with relying on a nuclear-armed ally rather than seeking their own arsenals, despite perennial concerns over the credibility of extended deterrence. Japan is a classic example: It has considered developing nuclear weapons at various points since the 1950s but has concluded it’s better off relying on the United States than taking the risk of producing an arsenal of its own.

Other states, such as Sweden and Germany, over time bought into the norm against developing nuclear weapons. In some cases, such as Argentina and Brazil in the 1980s, there are domestic divisions over the utility of nuclear weapons, and factions end up splitting the difference by developing civilian nuclear technology without developing an arsenal right away. For countries highly dependent on the international economy, such as Taiwan or the United Arab Emirates, the potential costs of going nuclear may outweigh the security benefits.

Even countries that decide they want nuclear weapons to protect themselves against (or coerce) an adversary often fail to acquire them. Authoritarian states in particular tend to mismanage their nuclear programs, as the cases of Iraq and Libya illustrate.

All this evidence aside, one could still argue there is something unique about the current conflict in Ukraine that would overwhelm these historical dynamics. But this is far from the first time a nuclear-armed state has attacked a nonnuclear state, nor is it the first time a nuclear state has used its nuclear arsenal to backstop coercion or aggression.

In 1967, Israel launched an attack that resulted in the occupation of the Golan Heights, Gaza Strip, West Bank, and Sinai Peninsula. Israel reportedly considered unveiling its recently acquired nuclear arsenal if necessary to stave off defeat. During the Vietnam War, then-U.S. President Richard Nixon attempted to use veiled nuclear threats to coerce the Soviets into pressuring the North Vietnamese on peace negotiations. Pakistan has repeatedly used its nuclear arsenal as a cover for aggressive actions against India. Russia annexed Crimea and occupied parts of eastern Ukraine in 2014 while knowing it could use its nuclear arsenal if necessary to deter outside intervention.

In none of these cases of nuclear-backed aggression did anything approaching a cascade of proliferation result. Nor did the regime-change operations against Iraq and Libya—countries that had given up their nuclear weapons programs under pressure—lead to a rush for the bomb (though they may have helped convince North Korea to hold on to what it has).

Is it possible that some countries might rethink their security options as a result of what happens in Ukraine? Absolutely. But countries are just as (or perhaps more) likely to conclude that they are better off finding or getting closer to a nuclear-armed ally—another important form of protection Ukraine lacked, being outside of NATO—as they are seeking their own arsenals. Even if one or two countries decide to seek a bomb, they will have to overcome substantial obstacles to successfully acquire one.

There are good strategic reasons to hope Ukraine prevails—for instance, it could help reinforce the norm against territorial conquest and possibly reduce the odds of future Russian aggression—but the case should not rest on averting a cascade of proliferation that is unlikely to materialize in any event.