The Advisory Opinion on the Legality of Nuclear Weapons

On the 5th of February 2026, the Strategic Arms Reduction Treaty signed in 2010 by the US and Russia (known as the New START treaty) will expire.¹ This treaty is the successor to a series of treaties signed by the US and the USSR which aimed to control the deployment of nuclear weapons.¹ With its expiration, there is no longer a single bilateral arms control agreement between Russia and the US, which had led to fears of a new nuclear arms race.

In 1994, the General Assembly (UNGA) adopted an resolution that requested an advisory opinion on the legality of the threat or use of nuclear weapons. The following is the text of the relevant section of that resolution²:

“The General Assembly, conscious that the the continuing existence and development of nuclear weapons pose serious risks to humanity, mindful that States have an obligation under the Charter of the United Nations to refrain from the threat or use of force against the territorial integrity or political independence of any State” with reference to several past resolutions of the General Assembly which are discussed in the section below which declared that the use of nuclear weapons would violate the UN Charter and be a crime against humanity, in light of progress, with regard to prohibition and elimination of other weapons of mass destruction, with reference to other conferences that have taken place about nuclear weapons and relevant international agreements, asks the following question for the ICJ to answer “Is the threat or use of nuclear weapons in any circumstance permitted under international law?”

This blog post discusses the response to the ICJ to that question in light of the fact that the opinion is likely to become more relevant in the current era, where progress regarding nuclear de-proliferation negotiations may begin to be reversed, despite that the “serious risk to humanity” that such weapons pose.²

The ICJ opinion

The court first had to address whether it had the jurisdiction to answer the question from the UNGA. They determined that they could answer the question so they moved onto establish what law was applicable to the question and came to a final determination as to the legality of nuclear weapons in light of the applicable law.

The general position in international law is that states can act in an unrestricted fashion because international law provides them with wide discretion. Therefore prohibitions at law, from any of the sources of law under international law, must be explicit. As a result, the court was seeking law that explicitly endorsed or prohibited the use of nuclear weapons in establishing the answer to the question sent to them by the UNGA.

Applicable law:

International Human Rights law:

Some states have argued Article 6 of the ICCPR which states ‘Every human being has the inherent right to life. This right shall be protected by law. No one shall be arbitrarily deprived of his life’³ prohibits the use of nuclear weapons. Others argued that the ICCPR was never intended to regulate the use of nuclear weapons and some went so far as to argue the ICCPR did not apply during wartime.

The court responded by stating the ICCPR does not cease to take effect in wartime. Insofar as a state has not derogated from the treaty in a state of emergency, it remains in effect. Even if such a derogation was to occur as the result of a war, the right to life is non derogable, meaning it remains in full effect even during a wartime state of emergency. The meaning of arbitrary deprivation of life in wartime will be governed by the law of war, IHL, instead of international human rights law.

Other states argued that the genocide convention prohibits the use of nuclear weapons because the effects of such weapons would naturally effect persons protected under that convention and intent to destroy a group can be inferred from the nature and long-term effects of such weapons. The court rejected this argument stating that an ordinary use of a nuclear weapon alone would not violate the genocide convention. However, if a nuclear weapon was to be used to commit the crime of genocide, this would be prohibited.

Multiple states also argued that the law of war’s prohibition on damage to the environment prohibits the use of nuclear weapons. They citied sources of law under both international human rights law and international humanitarian law. For example, some states relied on the 1977 additional protocol to the Geneva Conventions states ‘methods or means of warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment’.⁴ Other states argued that this area of law did not apply in wartime, did not apply to them or was not a binding source of law. In their view, given the lack of mention of nuclear weapons and wartime, it would undermine the stability of international law for this requirements to apply to this question.

The court agreed that such treaties may not completely prohibit the use of nuclear weapons, but their requirements must be taken into account when assessing whether the use of a nuclear weapon is permissible under the law of war, via the requirements of neccessity and proportionality. Furthermore, for states that are signatories of the 1977 protocol above, this obligations are particularly important.

The law on the use of force:

Article 2(4) of the UN charter prohibits a threat to use or the use of force in all cases aside from self defence under Article 51 or in cases where its authorised by the UN security council. The right of self defence is how many states justify recourse to nuclear force. However, the court noted that this right is mediated by principles of IHL, in particular proportionality and the profound risks of using nuclear weapons must be part of the assessment as to whether their use is lawful under the UN Charter.

With regard to the use of nuclear weapons in response to another state using them, it is clear that armed reprisals in peacetime are unlawful and in wartime, they are mediated by international humanitarian law and in particular the principles of necessity and proportionality.

With regard to the policy of deterrence, whether that would be a threat to use force depends on the circumstances. If the reason for stating readiness to use such weapons is unlawful, the threat to do so would be the same.

Custom of international law:

States that believe there is a customary prohibition on the use of nuclear weapons highlight the non use of nuclear weapons since 1945 but others claim this is merely because the circumstances in which states may use nuclear weapons have yet to arise. States also cite other UNGA resolutions in support of such a custom, but in the courts view the vote counts in favour of such resolutions indicate that there is insufficient opinio juris (states believing they are bound by a rule because it is law) practice for a custom to have emerged. This is partly because of the reliance on the policy of deterrence even though the court did not rule on the legality of this policy explicitly.

International humanitarian law:

The use of nuclear weapons is not explicitly unlawful under IHL, even under the various sources of law that prohibit other weapons of mass destruction (WMDs). This is because the pattern in this area of law is that each category of WMD is prohibited via its own treaty, which has yet to occur for nuclear weapons.

However, there are a series of treaties that regulate certain aspects of the process to make, the possession of nuclear weapons and several non proliferation treaties. Many states argue that these treaties alongside other relevant security council and general assembly resolutions form a rule of international law prohibiting the use of nuclear weapons. The court disagreed and states these treaties foreshadow a future prohibition on the use of nuclear weapons, but given that has yet to be addressed explicitly, no such rule exists today.

The key principles applicable from this area of law have general application via the 1977 protocol. The first is to protect civilians and civilian objects and the second is to prevent unnecessary harm and suffering of combatants. The second principle means even during war when targeting a lawful object, states do not have a free choice as to their use of weapon. Such rules must be followed by all states, even if they have not ratified the additional 1977 protocol from which they take effect generally. This was first recognised in the Nuremberg trials in 1945. The court found that these principles apply to the use of nuclear weapons, particularly in light of the fact they were developed long after many of these principles became law in the law 19th and early 20th century.

Ultimately, the court felt it could not state that IHL prohibited the use of nuclear weapons in all cases, particularly where a states survival was at stake, given the importance of self defence under Article 51 in international law. However, the court did also state ‘methods and means of warfare, which would preclude any distinction between civilian and military targets, or which would result in unnecessary suffering to combatants, are prohibited. In view of the unique characteristics of nuclear weapons, to which the Court has referred above, the use of such weapons in fact seems scarcely reconcilable with respect for such requirements’.

Nuclear de-proliferation treaties:

The advisory opinion ends with the court confirming the validity of and binding character of the various treaties that seek to put an end to possession of nuclear weapons by any state. This is an obligation that all states must co-operate to achieve and such co-operation must occur in good faith.

Ultimately, this opinion merely confirms that nuclear weapons are not inherently unlawful. Rather they like many other weapons can be used assuming the use of force in question is lawful under the Charter of the UN and IHL. This is on one hand a conservative opinion from the court, likely because of the prominence of states that possess nuclear weapons and the prominence of the policy of deterrence. This may appear disappointing and frankly it is- in particular the courts reluctance to rule on the policy of deterrence leave that key question unanswered. However, in stating that the principles of distinction between civilian and non civilian targets, proportionality and necessity is likely to bar the use of nuclear weapons in nearly all circumstances. Furthermore, this opinion is over two decades old and while the policy of deterrence remains with us, the law has evolved to bring us closer to a world without such weapons.

This is the context in which the new START treaty expired, a context in which the use of nuclear weapons would be illegal in nearly all circumstances. But frankly, a prohibition in law alone is not what has prevented the use of these weapons since 1945, it is likely that the fear of the consequences is what has prevented states from using them.