International Law

Part 2

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An international treaty may become binding on non-parties under various circumstances, for example --

    When the treaty rule reproduces an existing rule of customary law, the rule will be clarified in terms of the treaty provision. A notable example is the Vienna Convention on the Law of Treaties 1969, which was considered by the International Court of Justice (ICJ) to be law even before it had been brought into force.

    When a customary rule is in the process of development, its incorporation in a multilateral treaty may have the effect of consolidating or crystallizing the law in the form of that rule. It is not always easy to identify when this occurs. Where the practice is less developed, the treaty provision may not be enough to crystallize the rule as part of customary international law.

    Even if the rule is new, the drafting of the treaty provision may be the impetus for its adoption in the practice of states, and it is the subsequent acceptance of the rule by states that renders it effective as part of customary law. If a broad definition is adopted of state practice, the making of a treaty would fall within the definition.

Alternatively, it is possible to regard the treaty as the final act of state practice required to establish the rule in question, or as the necessary articulation of the rule to give it the opinio juris of customary international law.

International custom

Article 38(1)(b) of the ICJ Statute refers to "international custom" as a source of international law, specifically emphasizing the two requirements of state practice plus acceptance of the practice as obligatory or opinio juris sive necessitatis (aka "opinio juris") –

the conviction of States that the consistent practice is required by a legal obligation.

State practice

When examining state practice to determine relevant rules of international law, it is necessary to take into account every activity of the organs and officials of states that relate to that purpose. The principal means of contribution to state practice for the majority of states will be at meetings of international organizations, particularly the UN General Assembly, by voting and otherwise expressing their view on matters under consideration. Moreover, there are circumstances in which what states say may be the only evidence of their view as to what conduct is required in a particular situation.

The notion of practice establishing a customary rule implies that the practice is followed regularly, or that such state practice must be "common, consistent and concordant." Given the size of the international community, the practice does not have to encompass all states or be completely uniform. There has to be a sufficient degree of participation, especially on the part of states whose interests are likely be most affected, and an absence of substantial dissent.