Customary international law plays an important role in the applications recently submitted by the Marshall Islands to the International Court of Justice (ICJ), particularly in those against the nuclear weapon states not parties to the Treaty on the Non-proliferation of Nuclear Weapons (NPT). Has Article VI of this treaty become binding on all states even beyond the NPT, as argued by the applicant?
Customary international law, which is generally non-written, is created by the convergence of two elements: practice by a sufficiently representative number of states and other subjects of international law (for instance, international organizations) and ‘evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it’ (to use the ICJ’s words in, among others, the North Sea Continental Shelf Judgment) or at least by social, political or economic exigencies (opinio juris ac necessitatis). The fact that a treaty has been almost universally ratified is not, on its own, sufficient evidence of its customary status. To reach this conclusion, one has to look at the practice and opinio juris of the states not parties to the treaty: my feeling is that Israel, India, Pakistan and North Korea are going exactly in the opposite direction. Furthermore, even certain states parties (i.e., the five nuclear weapon states) have conducted themselves and expressed views that are difficult to reconcile with the alleged customary nature of Article VI.
It is true that universality of practice and opinio is not necessary for the formation of customary international law. However, it is not because there are a few states against in spite of a significant majority in favour that a custom corresponding to Article VI cannot be considered formed, but because the contrary practice and opinio come from those states that have nuclear weapons. To be clear, I am not referring to the doctrine of the persistent objector: I am not arguing that a custom in force is not applicable to certain states. My point is rather that a customary norm cannot sediment unless the practice and opinio juris of the specially interested states support this. The International Law Association’s 2000 Report on the formation of customary international law confirms that the extensive character of state practice is more a qualitative than a quantitative criterion: ‘if all major interests (“specially affected States”) are represented, it is not essential for a majority of States to have participated (still less a great majority, or all of them)’ (p. 26). Specially affected states are those that had the opportunity to engage in the relevant practice. It is difficult to see, then, how Article VI can become customary international law without considering the practice and opinio juris of those states that possess nuclear weapons and to which the provision is primarily addressed. The fact that Article VI does not reflect customary international law is confirmed in the ICJ’s 1996 Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, where the Court states that the obligation ‘formally concerns the 182 States parties to the Treaty on the Non-Proliferation of Nuclear Weapons’.
It should be incidentally noted that, if Article VI is only a treaty obligation and not also a customary provision, it would be at best an obligation erga omnes partes, and not erga omnes as too hastily argued by the applicant. In other words, it is an obligation assumed towards the group of the states parties collectively considered, but not towards the international community as a whole.