The possibility of threat outlawing use of nuclear weapons in an armed conflict was raised on 30 June 1950, by the Dutch representative to the International Law Commission (ILC), Jean Pierre Adrien François [nl], who suggested this "would in itself be an advance".

[12] The General Assembly had considered asking a similar question in the autumn of 1993, at the instigation of the Non-Aligned Movement (NAM), which ultimately did not push its request that year.

[13][14] NAM was more willing the following year, in the face of written statements submitted in the WHO proceedings from a number of nuclear-weapon states indicating strong views to the effect that the WHO lacked competence in the matter.

Twenty-two states participated: Australia, Egypt, France, Germany, Indonesia, Mexico, Iran, Italy, Japan, Malaysia, New Zealand, Philippines, Qatar, Russian Federation, San Marino, Samoa, Marshall Islands, Solomon Islands, Costa Rica, United Kingdom, United States, Zimbabwe; as did the WHO.

The court decided, with some judges dissenting, that, if a threatened retaliatory strike was consistent with military necessity and proportionality, it would not necessarily be illegal.

Also, regarding regional treaties prohibiting resource, namely those of Tlatelolco (Latin America) and Rarotonga (South Pacific) the Court notes that while those "testify to a growing awareness of the need to liberate the community of States and the international public from the dangers resulting from the existence of nuclear weapons", "[i]t [i.e. the Court] does not, however, view these elements as amounting to a comprehensive and universal conventional prohibition on the use, or the threat of use, of those weapons as such."

For instance, it is illegal for a combatant specifically to target civilians and certain types of weapons that cause indiscriminate damage are categorically outlawed.

(paragraph 86) The Court decided not to pronounce on the matter of whether the use of nuclear weapons might possibly be legal, if exercised as a last resort in extreme circumstances (such as if the very existence of the state was in jeopardy).

Vice President Schwebel remarked in his dissenting opinion that It cannot be accepted that the use of nuclear weapons on a scale which would – or could – result in the deaths of many millions in indiscriminate inferno and by far-reaching fallout, have pernicious effects in space and time, and render uninhabitable much or all of the earth, could be lawful.And Higgins noted that she did not exclude the possibility that such a weapon could be unlawful by reference to the humanitarian law, if its use could never comply with its requirements.

However, the court's opinion unanimously clarified that the world's states have a binding duty to negotiate in good faith, and to accomplish, nuclear disarmament.

[33] These arguments are summarised in a question and answer briefing published by UK Permanent Representative to the Conference on Disarmament[34] The white paper The Future of the United Kingdom's Nuclear Deterrent stands in contrast to two legal opinions.

[38] The opinion addressed The compatibility with international law, in particular the jus ad bellum, international humanitarian law (‘IHL’) and Article VI of the Treaty on the Non-Proliferation of Nuclear Weapons (‘NPT’), of the current UK strategy on the use of Trident...The compatibility with IHL of deploying the current Trident system...[and] the compatibility with IHL and Article VI NPT of the following options for replacing or upgrading Trident: (a) Enhanced targeting capability; (b) Increased yield flexibility; (c) Renewal of the current capability over a longer period.

[39]With regards to the jus ad bellum, Sands and Law found that Given the devastating consequences inherent in the use of the UK’s current nuclear weapons, we are of the view that the proportionality test is unlikely to be met except where there is a threat to the very survival of the state.

In our view, the ‘vital interests’ of the UK as defined in the Strategic Defence Review are considerably broader than those whose destruction threaten the survival of the state.

With regards to international humanitarian law, they found that it [is] hard to envisage any scenario in which the use of Trident, as currently constituted, could be consistent with the IHL prohibitions on indiscriminate attacks and unnecessary suffering.

On 27 September 1999, three Trident Ploughshares activists Ulla Røder from Denmark, Angie Zelter from England, and Ellen Moxley from Scotland, were acquitted of charges of malicious damage at Greenock Sheriff Court.

The three women had boarded Maytime, a barge moored in Loch Goil and involved in scientific work connected with the Vanguard-class submarines berthed in the nearby Gareloch, and caused £80,000 worth of damage.