… As far as I see it, the illegality of deployment of nuclear weapons on the Trident is based on the following:
The use of Trident weapons would be illegal in armed conflict as they are of a nature as to be unable to be used without violating international humanitarian law,
The Court concluded that “If the envisaged use of force is itself unlawful, the stated readiness to use it would be a threat prohibited under Article 2, paragraph 4. (Note that the threat to use is prohibited under Article 2 (4), which is applicable at all times. Thus, the UK argument that humanitarian law only applies in armed conflict is irrelevant with respect to the threat of use. It is only applicable to the use, and once nuclear weapons are used there is by definition a situation of armed conflict in which humanitarian law applies),
Trident weapons are deployed under a policy of “stated readiness to use.”
There is an additional argument which is that the deployment fails to meet the criteria of “extreme circumstance when the very survival of a state is at stake,” so that even if the Trident warheads were replaced with very small, precisely targettable warheads, according to the ICJ decision the deployment would not be permissible unless there was such an extreme circumstance.