What We Don’t Know About Nuclear Deterrence Could Hurt Us
French President Emmanuel Macron’s recent speech at the Sorbonne on his vision for the European Union, as well as his follow-up interview with The Economist, have attracted a lot of attention—as Macron intended. Never one to mince his words, Macron warned that “Europe can die” and expressed a sense of urgency in defeating Russia in Ukraine, even to the point of refusing to rule out sending French troops there in support of Kyiv.
While these comments made headline-grabbing news, another topic he touched on merits further attention: nuclear weapons.
In his speech at the Sorbonne, Macron stated, “Nuclear deterrence is at the heart of France’s defense strategy. It is therefore an essential element in the defense of the European continent.” In the Economist interview, Macron made his point more explicitly, saying that he was prepared to make France’s nuclear weapons available to protect the whole of Europe. Macron even referenced former French President Francois Mitterrand, and rightfully so. Back in the late 1980s, Mitterrand assured his West German counterpart, then-Chancellor Helmut Kohl, that France’s arsenal could be “the nuclear deterrent of Europe” in a bid to convince him to reject U.S. forward-deployed nuclear weapons on the continent.
If Macron’s comments weren’t enough to raise the nuclear stakes in Europe, Russia announced it will conduct a series of drills to simulate using so-called tactical or battlefield nuclear weapons in Ukraine. The drills were a direct response to Macron’s suggestion of putting French boots on the ground in Kyiv and yet another instance of Russian nuclear saber-rattling in order to dissuade further Western support to Kyiv.
All of this raises a fundamental question, one that goes beyond Macron, France, Russia and European security: Are nuclear weapons actually an effective deterrent? Just a few weeks ago, following a large-scale missile and drone attack by Iran on Israel, disarmament activists highlighted how Iran’s behavior—namely, its willingness to attack a country known to possess nuclear weapons—seemed to belie the idea that the possession of nuclear weapons facilitates deterrence.
Nuclear weapons states may indeed be dissuaded from launching massive nuclear attacks against one another, knowing that doing so would invite an equally massive response. This extreme scenario of “mutually assured destruction” admittedly worried many during the Cold War. But what about a scenario far below this threshold? What if, for example, Putin were to launch a nuclear strike on Ukraine using a single low-yield weapon? Would the United States, which is the primary provider of NATO’s “nuclear umbrella,” be willing to authorize an in-kind retaliatory strike? The doubts such a scenario raise make it seem perfectly reasonable to question the deterrent efficacy of nuclear weapons.
Overall, assessing the deterrence credibility of nuclear weapons, or deterrence in general, is extremely difficult. That is because we never observe deterrence. We only ever observe the failure of deterrence. When Russia attacked Ukraine, both in 2014 when it initially took the Crimean Peninsula and in 2022 when it launched a full-scale invasion of the country, it was clearly not deterred.
Overall, assessing the deterrence credibility of nuclear weapons, or deterrence in general, is extremely difficult. That is because we never observe deterrence. We only ever observe the failure of deterrence.
But suppose a timeline in which Russia had chosen not to attack Ukraine in either instance. It would be hard to know why Moscow had made that decision. It might have been due to fears of a retaliatory strike by some NATO countries—that is, deterrence. After all, NATO had formally guaranteed in 2008 that Ukraine would one day become a member of the alliance. And some NATO members, through the 1994 Budapest Memorandum, had an obligation to protect its territorial integrity.
But in such a scenario, Russia may have refrained from attacking Ukraine simply because Moscow, and specifically Putin, had no desire to seize Ukraine’s territory, at least not at that time. The problem is that fear of retaliation and lack of interest are often impossible to distinguish through observing a state’s behavior. Formally, scholars of international politics refer to this as “selection effects.”
Selection effects make assessing nuclear deterrence difficult enough. But there is another difficulty in assessing the deterrent value of nuclear weapons, one that is specific to nuclear weapons: They have only ever been used in war twice. The dropping of the atomic bombs on Hiroshima and Nagasaki at the end of World War II were tragedies, but they are tragedies that have not been repeated. Nina Tannenwald famously pointed to what she called the “nuclear taboo”—the idea that the weapons’ massive and indiscriminate destructiveness put their use beyond the moral pale—to explain why that has been the case.
What is good news for humanity, however, is bad news for science. The lack of historical cases since 1945 makes it difficult to know the exact conditions under which a nation’s leaders would go through with launching a nuclear weapon. We know that leaders have considered using nuclear weapons as an option during crises, and that humanity has come uncomfortably close to the actual launching of nuclear weapons. But rather than their nonuse pointing to the weapons being taboo, it could simply be due to dumb luck.
This indeterminacy over nuclear weapons’ lack of use matters, as it means that assumptions are often shaping our understanding of world politics. Some observers cite the fact that the world has avoided nuclear war since 1945 as evidence in favor of nuclear weapons serving as a deterrent. Some go so far as to claim nuclear weapons are responsible for a “long peace” and even the “decline of war.” Some, such as the prominent international relations scholar Kenneth Waltz, claim that the lack of nuclear use between the U.S. and the former Soviet Union during the Cold War suggests that we should actually be encouraging other nations to acquire the bomb and create a world in which everyone effectively deters everyone else.
But just as the nonuse of nuclear weapons in war since 1945 could be due to luck, the so-called long peace could be due to more than just nuclear weapons. The post-Cold War era witnessed a proliferation of democracy and expanded trade. It saw the adoption of normative and legal prohibitions against the use of force. In other words, the long peace is “overdetermined” and, hence, it is difficult to identify exactly to what degree it is actually due to effective nuclear deterrence compared to all of these additional factors.
Related to their being multiple causes of peace, there are also multiple causes of deterrence. When countries try to deter one another, nuclear weapons are rarely the only factor in play. Even when scholars have attempted to identify in a more limited and localized sense the effect of nuclear weapons in tipping a balance of power and creating deterrence, it is again difficult to discern the effect of the nuclear weapons from other factors that enhance deterrence. Consider North Korea. It is unclear whether the possession of a nuclear bomb is necessary for North Korea’s security, given that it has a large military amassed on its border with the South and tens of thousands of conventional missiles targeting Seoul, just 30 miles away. The onset of war on the Korean Peninsula would be devastating—and therefore to be avoided—even if Pyongyang had never acquired the ultimate weapon.
In sum, the deterrent value of nuclear weapons is unproven and in many ways unprovable, ultimately making them a gamble. Macron might place nuclear weapons at the center of French and European security, and Putin might rely on them to scare the West. But in both instances, they are gambling that others will back down over a fear of nuclear weapons’ destructive power. That bet might be correct. But it is a risky one, because it is difficult to know if nuclear weapons are truly an instrument of terror, an instrument of peace through terror or just pointless.
Paul Poast is an associate professor in the Department of Political Science at the University of Chicago and a nonresident fellow at the Chicago Council on Global Affairs.