Lawyers often use customary international law broadly to refer to all generally applicable non-treatybased international law. In a narrower and more technical sense, scholars mean those rules of nonwritten international law that express a consistent and general pattern of state behavior that has continued for some duration of time and that is accompanied with the conviction that the behavior is required as a matter of law (opinio juris).

Customary law is one of the three primary sources of international law laid out in the Statute of the International Court of Justice (art. 38). The other two are treaties and “general principles of law recognized by civilized nations.” It is precisely because of the somewhat dubious nature of the latter source that most or all of non-written ...

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