Updated: Jun 20, 2020

The 1996 advisory opinion of the International Court of Justice (ICJ) was the culmination of a decades-long debate on the legality of nuclear weapons. In recent years, it has shaped how

international law is invoked by the initiative focused on the humanitarian impacts of nuclear

weapons use and served as a foundation for the nuclear disarmament cases brought by the Marshall Islands in the court.

The legality of the use of nuclear weapons had been considered by the UN General Assembly since 1961 when the body adopted Resolution 1653 by a divided vote. The resolution declared that such use “is contrary to the rules of international law and to the laws of humanity.” But the

General Assembly’s 1994 resolution requesting the ICJ “urgently to render” an advisory opinion on the matter set in motion an entirely different, extraordinary process. The General Assembly asked the court to opine on the following question: “Is the threat or use of nuclear weapons in any circumstance permitted under international law?” This put the issue before the judicial branch of the United Nations, the highest court in the world on questions of international law.

Several nuclear-weapon states chose to defend the lawfulness of using nuclear weapons in extended arguments to the court. Russia, the United Kingdom, and the United States argued that although nuclear arms, like other weapons, are subject to the law of armed conflict, whether their use would be lawful or unlawful would depend on the circumstances. France contended that absent a specific prohibition, the weapons may be employed in the exercise of the right of self-defense. States not reliant on nuclear weapons, plus Australia, a country closely aligned with a nuclear-armed state, argued that the effects of nuclear explosions are inherently uncontrollable and indiscriminate and that the use of such weapons is therefore unlawful in all circumstances. Over two weeks of dramatic hearings in November 1995, 22 states made oral arguments, most after also submitting written arguments, and another 23 made written submissions only. Altogether, 45 states participated, the largest number to do so in ICJ proceedings to that date. Civil society also played a role. More than 700 groups worldwide had joined together in the World Court Project to support the General Assembly’s request for an opinion and to publicize the initiative.

In reaching its conclusions, the court considered a range of legal issues relating to threat or use of nuclear weapons, not all of which can be reviewed here. One significant finding was that a customary norm of non-use of nuclear weapons has not arisen out of the practice of non-use

since World War II due to states’ continued assertion of doctrines of deterrence that contemplate resorting to nuclear arms. Another way that impacts on the environment must be taken into account in assessing whether an attack complies with the law of armed conflict. Also important was the court’s examination of the legal status of threats. It found that “if the use of force itself in a given case is illegal—for whatever reason—the threat to use such force will likewise be illegal.” But the heart of the court’s analysis and of the arguments made by states concerns the application of the law of armed conflict to the use of nuclear weapons.

The court laid particular emphasis on two elements of what is commonly referred to as international humanitarian law: the prohibition of causing unnecessary suffering to combatants and the principle of distinction between combatants and non-combatants. Under the principle of distinction, the court stated categorically, “States must never make civilians the object of attack and must consequently never use weapons that are incapable of distinguishing between civilian and military targets.” The court did not refer to another aspect of the principle of distinction, the prohibition of attacks on military targets expected to cause excessive incidental harm to civilians and civilian objects when weighed against the expected military advantage. Perhaps the court considered it to be inapplicable in the nuclear realm, where collateral casualties can number in the tens or hundreds of thousands.

The court said that the rules of international humanitarian law highlighted reflect "elementary considerations of humanity” and are “fundamental.” Moreover, they are “intransgressible,” an innovative term that must mean that the rules are not to be violated whatever the circumstance. The court further said that for a use of force in response to an armed attack to be proportionate, a requirement for the lawful exercise of the right of self- defense, it must also comply with international humanitarian law. Although the court declined to address whether the legal doctrine permitting reprisals aimed at dissuading further unlawful acts by an enemy could justify the use of nuclear weapons in response to prior use, it stated that any reprisal must meet the requirement of proportionality. The court found that it was unable to assess the legality of use in marginal scenarios such as the use of low-yield nuclear weapons in remote areas, but stated that the nuclear-weapon states had failed to make the case for legality in such circumstances.

The thrust of the analysis is captured by the court’s finding that, in view of their “unique characteristics,” the use of nuclear weapons is “scarcely reconcilable” with the requirements of international humanitarian law. That finding has been taken up as a subsidiary theme by the initiative on the humanitarian impact of nuclear weapons.

Given its analysis of the application of international humanitarian law to the use of nuclear

weapons, what sense can be made of the court’s inability to determine the lawfulness or

unlawfulness of a threat or use of nuclear weapons in an extreme circumstance of self-defense

in which the very survival of a state is at issue? This provision has been the subject of harsh criticism, and most members of the court signaled their discomfort or disagreement with it in their separate statements accompanying the opinion. It seems incongruous that a court would declare itself unable to apply the law to an important issue in the matter before it. The provision left an opening for states relying on nuclear arms to claim that their policies of deterrence are lawful and that use could be lawful—an opening that they in fact seized. No similar clause exists in international law governing the use of force, the conduct of warfare, and the use of particular weapons. Further, the court’s finding arguably implies that international humanitarian law possibly could bend in extreme circumstances of self-defense, whereas the essence of that law is that it applies to all states, whether aggressor or defender, in all circumstances. Indeed, as already noted, the opinion elsewhere is abundantly clear that whatever the circumstance, the use of nuclear weapons must comply with international humanitarian law. States must “never use weapons that are incapable of distinguishing between civilian and military targets.”

The court’s reference to the policy of deterrence in justifying the finding suggests that the finding is as much or more about threat than it is about used. It appears to reflect the stark realities of threat and counter-threat at least implicitly faced by states when other potentially adverse states possess nuclear weapons. All of this points toward the comprehensive prohibition and elimination of nuclear arms as the only real solution to the dilemmas posed by their existence, a subject addressed by the court in the final section of the opinion.

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