Custom, Power, and the Power of Rules: International Relations and Customary International Law

One of the concepts normally considered fundamental to the rule of law is that the law of any society must in principle apply to all its members. In national legal systems this concept may be understood in at least two ways. First, it is possible to understand law as being imposed from above by the State or sovereign, and as generally applicable to all citizens.¹ Secondly, it is possible to understand law as a multitude of bilateral relationships, either between individual persons or between individuals and the State.²

In international society there is no overarching sovereign, and international law has frequently been understood as involving a multitude of bilateral relationships between those entities which have international legal personality, i.e. predominantly States.³ Since there is no overarching sovereign at least some parts of this law do not necessarily need to apply to any one State, nor does this law have to apply in the same way to all States, that is, it does not need to be generalised. Instead, the application of rules of international law to a State is usually regarded as being dependent on that State's consent, which may be accorded either to specific rules, or to legal processes more generally.⁴ This consent operates bilaterally, as can be seen in the requirement of consent by States parties to reservations to multilateral treaties,⁵ and in the existence of special customary international law.⁶

It may appear that this bilateralist understanding of international law is incompatible with the existence of generally applicable rules, especially those principles which structure the international legal system.⁷ Simma has criticised bilateralism as being 'a barrier in the way of stronger solidarity in international law' and argued that jus cogens and erga omnes rules

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