Nuclear weapons, including threats of using them, have returned to the centre of international politics. From Putin’s repeated nuclear threats during Russia’s war against Ukraine to recent remarks by Trump attempting to use nuclear weapons as instruments of coercive diplomacy in the war in Iran, leaders of nuclear-weapon states have brought dangerous escalatory rhetoric back into everyday politics.

Yet 30 years ago today, the International Court of Justice (ICJ), in its landmark advisory opinion on the “Legality of the Threat or Use of Nuclear Weapons”, drew a direct line between such threats and the potential use of nuclear weapons, arguing that the legality of a nuclear threat depends on whether the use of nuclear weapons would itself be lawful.

The landscape has changed dramatically since the advisory opinion was delivered in July of 1996. Nuclear deterrence has returned to the centre of international security, arms control agreements have eroded, and geopolitical rivalry has intensified. At the same time, humanitarian concerns surrounding nuclear weapons have gained unprecedented prominence, culminating in the adoption and entry into force of the Treaty on the Prohibition of Nuclear Weapons (TPNW).

In this environment, the ICJ’s advisory opinion remains relevant as a reminder of a legal common ground: nuclear weapons are subject to international law and nuclear-weapon states need to abide by their disarmament obligations.

Yet the advisory opinion also underscored a tension that has continued to define nuclear politics ever since: international law places constraints on nuclear weapons and their use, while nuclear-weapon states continue to argue that their possession and potential use of weapons of mass destruction remain necessary for national security. The Court rejected the idea that nuclear weapons exist outside the bounds of legal scrutiny, but it also stopped short of declaring them unlawful in every circumstance.

Three decades on, as the legal and political landscape increasingly evolves in opposite directions – more favourable to prohibition, but also more resistant to it – that unresolved tension remains at the centre of debates over deterrence and disarmament.

What the ICJ decided in 1996

The advisory opinion is often remembered for what it did not decide: the Court did not declare nuclear weapons to be either always illegal or clearly lawful. Yet the ruling contained several provisions that remain highly consequential.

Importantly, it confirmed that nuclear weapons are not beyond the reach of international law. While the advisory opinion noted that no existing treaty (at the time) comprehensively prohibited the use of nuclear weapons, this does not mean that they exist in a legal vacuum.

The Court made clear that any use of nuclear weapons would still have to comply with the UN Charter, as well as the fundamental rules of international humanitarian law – meaning that the principles of distinction, proportionality, and the prohibition of unnecessary suffering would apply. For nuclear weapons, with their indiscriminate and long-term effects, meeting these requirements would present an extraordinarily difficult, if not impossible, hurdle to cross.

The Court went further on one point with direct bearing on today’s heightened nuclear rhetoric. It highlighted that, in accordance with the UN Charter, there is no difference between the legality of a threat to use and the legality of the actual use. In other words, if the use of nuclear weapons would be unlawful, then the threat to use them would be unlawful too. Given how difficult the Court found it to imagine nuclear use that meets the requirements of international humanitarian law, this finding leaves little room for nuclear threats of the kind now routinely made by the leaders of some nuclear-weapon states.

The Court also addressed the difficult question about whether a state “in an extreme circumstance of self-defence, in which its very survival would be at stake” could ever lawfully use nuclear weapons. While the Court found that any use would have to comply with international humanitarian law, it could not agree such compliance was impossible in every conceivable circumstance. It thus left unresolved the precise scenario on which nuclear-weapon states have traditionally relied to justify the continued practice of nuclear deterrence.

Finally, the advisory opinion reconfirmed that nuclear-weapon states have an obligation to pursue negotiations on nuclear disarmament in good faith. Unlike the Court’s discussion of the legality of nuclear use, its finding on disarmament was unanimous. More importantly, in the advisory opinion, disarmament was identified not simply as a distant aspiration or a political commitment, but as a legal obligation under Article VI of the Nuclear Non-Proliferation Treaty (NPT). Rather than simply assessing the legality of existing practices, the Court thus formulated a concrete, affirmative, and forward-looking obligation that required states to take action instead of merely maintaining the status quo.

Since 1996, these elements have become central arguments that have backed disarmament advocacy by many non-nuclear-weapon states. For these countries, as well as for civil society, the advisory opinion provided not only important legal and political support for disarmament, but also for the humanitarian arguments that later supported the negotiation of the TPNW.

Thirty years on: progress and backsliding

Today, the world looks both more committed to and less capable of achieving nuclear restraint. The fundamental disagreement over nuclear weapons that existed in 1996 has not disappeared. If anything, the divide has become more entrenched.

What’s more, the overall security environment has deteriorated significantly, providing nuclear-weapon states and their allies with an argument for recentring nuclear deterrence in their security policies. On the other hand, the humanitarian and normative landscape has also changed. Since the advisory opinion was issued, the humanitarian consequences of nuclear use and testing are receiving ever greater attention, including through discussions of the long-term health and environmental effects of nuclear weapons. Developments in human rights and environmental law have further strengthened arguments about the intergenerational consequences of nuclear weapons and the potential illegality of future nuclear weapons use. Most importantly, the TPNW created a new legal and political platform for states seeking to prohibit nuclear weapons. Many supporters underline that the TPNW reflects and contributes to the emergence of a customary prohibition norm – thus helping to close the legal gap left open by the advisory opinion.

Would the ICJ, asked the same question today, reach a different answer? This question has become even harder to answer, with a stronger humanitarian and legal framework pulling in one direction, and entrenched practices of deterrence pulling the other. But this is exactly why the ICJ’s advisory opinion still matters.

Taking the advisory opinion as a call for action

The advisory opinion’s greatest legacy is not that it resolved the debate over the (il)legality of nuclear weapons. Instead, it established a common legal reference point for debates about the relationship between nuclear weapons and international law, confirming that nuclear weapons are subject to international law and that disarmament is a legal obligation, not a political choice.

In the last 30 years, the legal and political tensions surrounding the advisory opinion have become more consequential: the humanitarian and legal arguments for prohibition have grown stronger, while reliance on nuclear deterrence has become more deeply entrenched. That is precisely why the anniversary of the ICJ’s advisory opinion should not only invite reflection, but prompt action.

For nuclear-weapons states, the advisory opinion remains a reminder that deterrence does not create a legal exception. Nuclear planning should therefore include a more systematic assessment of humanitarian consequences and legal obligations, rather than treating deterrence requirements as overriding considerations.

Nuclear-weapons states should also take the Court’s unanimous finding on disarmament more seriously. Article VI of the NPT cannot remain a rhetorical commitment repeated at Review Conferences while practical progress remains limited. Reviving meaningful arms control and disarmament dialogue is not simply a concession to critics of nuclear deterrence; it is part of the legal and political bargain that has sustained the NPT for decades. Fulfilling the ICJ’s call would simply require states to honour commitments they have already unanimously accepted.

The advisory opinion also casts recent nuclear rhetoric in a different light. If the legality of nuclear threats depends on the legality of the potential use of nuclear weapons, such threats cannot simply be dismissed as political messaging or deterrence signalling. Non-nuclear weapon states should therefore be more willing to confront nuclear threats, including when they come from nuclear-armed allies. Aggressive nuclear rhetoric should not become normalised simply because it is presented as part of deterrence policy.

The more consequential question is not whether enough states are ready to support the TPNW, but why nuclear deterrence remains so politically, strategically, and socially embedded among nuclear-weapons states and their allies. Until the security concerns raised to justify the growing salience of nuclear weapons are not effectively countered, legal arguments alone are unlikely to dislodge reliance on nuclear deterrence.

The European Leadership Network itself as an institution holds no formal policy positions. The opinions articulated above represent the views of the authors rather than the European Leadership Network or its members. The ELN aims to encourage debates that will help develop Europe’s capacity to address the pressing foreign, defence, and security policy challenges of our time, to further its charitable purposes.

Image credit: Denny van der Vaart