The role of usus and opinio in international humanitarian law

Usus and opinio, as elements of customary law, play a different role in a particular branch of international law, the humanitarian law of armed conflict.

By the same token, the requirement of opinio juris or opinio necessitatis may take on special prominence. As a result, the expression of legal views by a number of States and other international subjects about the binding value of a principle or a rule, or the social and moral need for its observance by States, may be held to be conducive to the formation of a principle or a customary rule, even when there is no widespread and consistent State practice, or even no practice at all, to back up those legal views.

Do customary rules need, at their birth, the support of all states?

At present, when they gradually crystallize in the world community, customary rules do not need to be supported or consented to by all States. For a rule to take root in international dealings it is sufficient for a majority of States to engage in a consistent practice corresponding with the rule and to be aware of its imperative need. States shall be bound by the rule even if some of them have been indifferent, or relatively indifferent, to it (one may think of the position of landlocked States, in the process of formation of the law of the sea), or at any rate have refrained from expressing either assent or opposition. That universal (express or implicit) participation in the formation of a customary rule is not required is evidenced by the fact that no national or international court dealing with the question of whether a customary rule had taken shape on a certain matter has ever examined the views of all States of the world.

Nevertheless, the existence today of so many international organizations to a great extent facilitates and speeds up the custom-creating process, at least in those areas where States are prepared to bring general rules into being.

Custom is by no means on the wane everywhere. There are at least three areas where it plays a significant role, and is indeed acquiring growing importance.

The first is in emerging economic interests such as, for example, those relating to the law of the sea. The rapid growth of new economic demands often cannot be as rapidly co-ordinated and regulated by treaties in this area as in others. This is because numerous conflicts between groups of States and the complexity of all the closely interrelated matters need to be taken into account. By contrast, solutions to specific issues, propounded by one or more States, may come eventually to satisfy the interests and needs of others and thus bring about the gradual emergence of customary rules.

Second, there is the area of major political and institutional conflicts, where new needs in the international community give rise to strong disagreements between 'States, and it may therefore prove extremely difficult to achieve regulation via treaty rules.

A third area where custom is relatively vigorous is the updating and elaboration of those parts of the body of customary law which newly independent States have considered to be more or less acceptable, although in need of some revision and clarification.

In other words certain parts of traditional law have been supplemented and elaborated upon by conventional rules carrying the imprint of general rules in spite of their being consecrated in treaties.