The Court interpreted the numerous GA resolutions on the question of nuclear weapons as doing no more than revealing ‘the desire of a very large section of the international community to take steps towards nuclear disarmament’ (para 73). It did not recognize that the GA has authority to enact international law. More importantly, in assessing the resolutions, it was unable to find a requisite opinio juris to that end, due to the inconsistent voting of the member States during their adoption—that is, in one instance State A voted in favour and, in another instance, voted against.

The case was initiated by Nicaragua and concerned the various activities of the United States in and against Nicaragua in breach of various rules of international law, including the prohibition of the use of force and non-intervention. Due to the US reservation to the optional clause in the ICJ Statute, the Court was forced to apply only customary international law. Although the use of force was regulated under customary law prior to the UN Charter, there was doubt as to whether the existence of conflicts and forcible interventions since 1945 gave rise to any sort of relevant and consistent practice.

The Court held that ‘it is not to be expected that in the practice of States the application of the rules in question should have been perfect, in the sense that States should have refrained, with complete consistency, from the use of force or from intervention in each other’s internal affairs. The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule . . . If a State acts in a way prima facie inconsistent with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than weaken the rule’ (para 186) (emphasis added).

It also held that the fact that the previously mentioned principles, recognized as such, have been codified or embodied in multilateral conventions does not mean that they cease to exist and to apply as principles of customary law, even as regards States that are parties to such conventions. Principles such as the non-use of force, non-intervention, respect for the independence and territorial integrity of States, and freedom of navigation continue to be binding as part of customary international law, despite the operation of provisions of conventional law in which they have been incorporated.