Nuclear What is the Role of International Law in Preventing the Use of Nuclear Weapons, Specifically by Russia Against Ukraine?

July 19, 2023
By Janina Dill | Perry World House

Janina Dill is the John G. Winant Professor of US Foreign Policy at the University of Oxford. This thought piece was written for the Perry World House workshop “The Future of Nuclear Weapons, Statecraft, and Deterrence after Ukraine.”

There are few documented occasions on which states have contemplated a nuclear strike. The actual use of nuclear weapons has thankfully been even rarer. This does not lessen the urgency of understanding how future nuclear use can be prevented and how international law can contribute to this aim. However, the small number of documented decisions on nuclear use means that investigating these decisions in detail may not yield much insight into the role that law can, in principle, play in constraining nuclear use. Similarly, we may learn little in this way about whether international law can prevent Vladimir Putin from launching a nuclear strike against Ukraine, the question that currently preoccupies us most. The best bet to answering it is to examine under what conditions international law is generally effective in constraining decision-making in international relations. We can identify the challenges to international laws being an effective constraint on future nuclear use by comparing this scenario to the ideal scenario in which international law typically attracts compliance.

International law, unlike domestic law, is not associated with the coercive power of a sovereign that reliably imposes sanctions whenever a subject violates the rules. States are both the sovereign and the subjects of international law, its creators, and the main addressees of its demands. States often conform to these demands, partly because they preferentially create and participate in regimes that align with their interests anyway. Of course, international law is particularly important when states may have reason to break a rule that applies to them. The crucial question is whether and when international law can add to the reasons a decision-maker contemplates for and against a course of action and potentially tip the balance of reasons against breaking the law. We should assume here that states only contemplate the use of a nuclear weapon when they have identified a weighty reason in favor of nuclear use. Presumably, they also see some costs. In that situation, can international law tip the balance of reasons against an attack?

What makes international law an effective constraint on state behavior suggests that, unfortunately, it faces significant challenges in playing a role in preventing nuclear use, specifically by Russia. Existing studies suggest that variables that predict when international law effectively constrains state behavior fall in three baskets: first, agent-related variables, i.e., some actors are more likely to consider law in their decision-making than others; second, contextual variables, i.e., some problems are structured in ways that make it more likely that states give weight to legal considerations; and third, variables about the law, i.e., some types of international laws are generally better at constraining state behavior than others. The variables that are associated with legal compliance across these three baskets are, with one exception, absent or weak in the nuclear context. Rather than giving up on international law, however, this insight should spurn us on to reform and improve international legal rules, specifically their determinacy and legitimacy. Moreover, the current weakness of international law gives us reason to articulate the moral case against nuclear use more effectively.  

Let’s start with agent-related variables. Variation in regime type has long been linked to variation in compliance with international law. Democracies are, all things considered, more likely to comply with international law. Regardless of how we precisely define democracy, there are enough nuclear-armed states that do not qualify, and Russia, which arguably currently poses the greatest nuclear threat, certainly does not. One reason for why regime type is linked to compliance with international law is the structure of decision-making. In autocracies decision-making is less shaped by bureaucratic processes that incorporate legal considerations, and the input of technical expertise, including, from lawyers. Moreover, extensive research has established that ordinary people prefer that their leaders comply with international law. Accountability at the ballot box and lobbying from domestic interest groups are well-studied reasons for decision-makers in democracies to give weight to international law. In general, democratic publics have been shown to impose costs on leaders that make public commitments, including legal commitments, and then renege on them. International law is significantly less likely to constrain a decision-maker such as Vladimir Putin, who incurs fewer audience costs, in less predictable ways.  

What makes international law an effective constraint on state behavior suggests that, unfortunately, it faces significant challenges in playing a role in preventing nuclear use, specifically by Russia.

If we acknowledge that in personalized dictatorships it may be an individual alone that decides on nuclear use, then we need to ask what we know about individuals’ propensities to comply with law. There is likely significant variation in the extent to which individuals see legality as its own end and in the extent to which they routinely consider or even defer to law. Indeed, elite surveys have shown that a cosmopolitan identity and liberal internationalist beliefs make leaders more likely to defer to international law. We do not know much about Putin’s disposition, but it is safe to assume that he has neither trait. Both agency-related factors that give international law a better chance of eliciting compliance, regime type and an individual decision-maker with liberal cosmopolitan beliefs, are hence absent in the scenario of a potential nuclear use by Russia against Ukraine.  

When it comes to contextual factors, we know that international laws have a better chance of attracting compliance if they demand actions that are predictably subject to reciprocity. For instance, in the context of the use of force, research suggests that states are more likely to treat prisoners of war in line with legal demands if the other side has prisoners of their own. More generally, as Beth Simmons famously showed, “governments commit to and comply with legal obligations if other countries in their region do so.” Deterrence, specifically under conditions of secure second-strike capabilities on both sides, harnesses this logic of reciprocity for the prevention of nuclear use. Scholarship on compliance with international law thus lends weight to the consensus in strategic studies that deterrence is a crucial constraint on nuclear use. This scholarship on the role of reciprocity in legal compliance, however, also implies that a clear commitment by other nuclear powers not to use nuclear weapons first against Russia would be conducive to  preventing a Russian nuclear strike, a point to which I will return below. Reciprocal deterrence favors compliance with international law.

At the same time, the question arises whether international law is really implicated in deterrence. Non-use of nuclear weapons is clearly a behavioral pattern, a tradition, likely an international norm, but is it a legal rule? The question of whether international law prohibits nuclear use in armed conflict is contested. There are three theories as to the sort of international law that typically attracts compliance. The first relates to the precision of the law: determinate laws are more likely to be obeyed. The second theory relates to the authority of law: laws perceived as legitimate generate obedience. A third theory relates to the institutionalization of legal demands: monitoring by international institutions is associated with increased compliance. Unfortunately, the contestability of the legal prohibition on nuclear use renders indeterminate the implications of international law for nuclear use and undercuts its authority. Contestability of whether and when international law prohibits nuclear use, in turn, is nurtured by the official legal positions of all nuclear states, including democracies. In short, international law on the use of nuclear weapons is weak. This is a choice.

Jutta Brunnée and Stephen Toope argue that “actors must be able to understand what is permitted, prohibited, or required by law – the law must be clear. Law should avoid contradiction, not requiring or permitting and prohibiting at the same time.” Lon Fuller made the point that determinate laws “inspire ‘fidelity’ of social actors to law.” Yet, international law has not produced a general and precise prohibition on nuclear use, and there is plenty of contradiction in the legal sources. Just over twenty years ago, the International Court of Justice famously stated that the use of nuclear weapons “seem[ed] scarcely reconcilable with respect” for the requirements of international humanitarian law (IHL). At the same time, the Court averred that it could not “reach a definitive conclusion as to the legality of the use of nuclear weapons by a State in an extreme circumstance of self-defence, in which its very survival would be at stake.” When the rules for the conduct of hostilities were fully codified in 1977, in the First Protocol Additional to the Geneva Conventions, several nuclear-armed states, including the United States and the United Kingdom, declared that the rules of the treaty did “not have any effect on and [did] not regulate or prohibit the use of nuclear weapons.” The United States subsequently refused to ratify the Protocol.

Nowadays, it is hardly contested that much of the Protocol has the standing of customary international law, binding even states that are not party to the treaty, and that an attack with nuclear weapons must comply with the rules for the conduct of hostilities set forth by IHL. The Obama administration clarified that nuclear targeting plans would be “consistent with the fundamental principles of the Law of Armed Conflict . . . [and] apply the principles of distinction and proportionality and seek to minimize collateral damage to civilian populations and civilian objects.” As a result, some types of nuclear use are clearly prohibited. For instance, deliberately targeting population centers violates IHL’s principle of distinction. As Scott Sagan and Allan Weiner show, counter-population targeting has not, however, disappeared from the nuclear doctrine of the United States. Moreover, some officials maintain that the United States could use nuclear weapons to target civilians by way of reprisal.

Non-use of nuclear weapons is clearly a behavioral pattern, a tradition, likely an international norm, but is it a legal rule?

In addition, the rules for the conduct of hostilities are indeterminate enough to leave room for contestation of what other principles besides distinction imply for nuclear use. In as much as nuclear-armed states are now able to dial down the blast radius of their nuclear weapons, scholars have been able to imagine a variety of scenarios in which a low-yield nuclear strike would be discriminating, and milder alternatives would be ineffective, so that nuclear use might be considered necessary. Lawyers have also argued that the incidental harm to be expected from a low-yield nuclear strike could be proportionate if the anticipated military advantage were particularly weighty. Whether a specific nuclear attack would be disproportionate and thus illegal will always be contestable due to the widely appreciated indeterminacy of a law that demands a projected “balance” between civilian harm and military advantage.

Does nuclear use not fall foul of the prohibition on unnecessary suffering and superfluous injury? It seems unlikely that even a low-yield nuclear strike would be permissible if its long-term consequences were considered when determining compliance with this principle. This, however, is contested. Similarly, controversy persists as to whether even nuclear weapons with a much smaller payload than those fatefully used in 1945 would inevitably “cause widespread, long-term and severe damage to the natural environment” and thus be prohibited. In short, the permissibility of nuclear weapons under IHL is contingent on the factual circumstances of the precise attack that a belligerent contemplates, i.e., the concrete consequences of a specific nuclear use, and how they stack up against very open-ended legal principles. The alternative to using the indeterminate principles for the conduct of hostilities to assess the legal permissibility of specific nuclear attacks would be an outright ban on the use of any nuclear weapon. The treaty that does as much, however, is not in force for any nuclear-armed state.

The refusal of nuclear-armed states to join the Nuclear Ban Treaty leaves international law on nuclear use not only less determinate than it could be, but it also undermines its authority. It is well established that states are more likely to comply when international law is widely deemed authoritative – compliance promises the mantle of legitimacy, while non-compliance incurs reputational costs. Widespread practice that is congruent with legal demands has been shown to add to the perceived legitimacy and authority of a legal rule. Deterrence, as practiced by nuclear-armed states, however, means consistently reaffirming one’s willingness to use nuclear weapons. Any attempt to derive a prohibition of nuclear use from international law is up against this contradictory practice, and therefore unlikely to gain the legitimacy that makes a rule authoritative.

Of course, nuclear-armed states have clearly articulated reasons for not joining the Nuclear Ban Treaty, for allowing tensions between their nuclear doctrines and applicable international law, and for consistently reaffirming their readiness to launch a nuclear strike. Upholding deterrence is the reason mentioned most frequently. As stated above, deterrence plays a crucial role in constraining nuclear use. Law should not undercut it. Is there a way to square the circle? Could nuclear states, specifically nuclear-armed democracies, throw their weight behind international law and make laws on nuclear use more precise and authoritative, without undermining deterrence? There are two avenues forward.

First, all nuclear-armed states should work toward adopting a “no-first-use” policy and give this rule legal standing. This would not undermine deterrence of a nuclear strike. It could weaken deterrence of a conventional attack in specific bilateral relations, a risk which could be managed by other means. At the same time, the benefits of a no-first-use policy, specifically its role in preventing unintended escalation, are well-appreciated. What is less appreciated is that refraining from a no-first-use policy weakens international law and adopting one strengthens a crucial compliance mechanism: reciprocity. Moreover, as mentioned, for international law to play a role in constraining nuclear use, we need to have a precise, clear-cut rule, one that is upheld in practice and has a chance to gain legitimacy. A categorical ban on first use could deliver that.

Second, nuclear states, particularly nuclear-armed democracies, should be less hesitant to make the moral case against nuclear use. Again, the fear of emboldening adversaries, worrying allies, and weakening deterrence has kept the governments of nuclear-armed democracies, except for the Obama administration, from systematically and officially acknowledging the potentially planet-ending moral catastrophe that would be nuclear use. Yet, any reasonable observer presumably understands the moral costs of a nuclear strike. Moreover, there is no evidence that actor A’s acknowledging the moral evil of nuclear use makes actor B less likely to believe actor A's threat of a retaliatory strike, though it might make first use less credible. Put differently, we lack empirical evidence that moral outrage about nuclear use undermines deterrence. At the same time, the moral case against nuclear weapons could lend urgency to articulating more precise international law and would lend legitimacy to a law against first use.

The statements made and views expressed in this article are solely the responsibility of the author.