Duhaime's Law Dictionary

Customary International Law Definition:

International law which does not have a treaty base but, rather, exists because of international custom.

Related Terms: International Law, Private International Law, Law of Nations, Opinio Juris

In R v NY, Justice of the Ontario Superior Court adopted this definition originally contributed by International Red Cross lawyer, Jean-Marie Henckaerts:

"The Statute of the International Court of Justice describes customary international law as a general practice accepted as law. It is widely agreed that the existence of a rule of customary international law requires the presence of two elements, namely State practice (usus) and a belief that such practice is required, prohibited or allowed, depending on the nature of the rule, as a matter of law (opinio juris sive necessitatis).

"It is of course axiomatic that the material of customary international law is to be looked for primarily in the actual practice and opinio juris of States.”

In a 2003 decision of the United States Court of Appeals, Justice Jose Cabranes presiding, Flores v Southern Peru Copper Corporation, the judge, at great length, described customary international law, no distinction being evident between this concept and the widely-held definition of international law, also known as the law of nations:

"Plaintiffs claimed that defendant's conduct violates the law of nations - commonly referred to as international law or, when limited to non-treaty law, as customary international law....

"We have consistently used the term customary international law as a synonym for the term the law of nations. The term customary international law echoes the earlier phrase sometimes used to describe the law of nations, the customary law of nations ... non-treaty-based law of nations as the the customary law of nations.

"(C)ustomary international law is composed only of those rules that States universally abide by, or accede to, out of a sense of legal obligation and mutual concern.

"First, in order for a principle to become part of customary international law, States must universally abide by it.... Customary international law includes only well-established, universally recognized norms of international law.... Of course, States need not be universally successful in implementing the principle in order for a rule of customary international law to arise. If that were the case, there would be no need for customary international law. But the principle must be more than merely professed or aspirational.

"Furthermore, a principle is only incorporated into customary international law if States accede to it out of a sense of legal obligation. Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation.

"Practices adopted for moral or political reasons, but not out of a sense of legal obligation, do not give rise to rules of customary international law.... Customary international law does not include those practices that States have adopted for moral or political reasons (as opposed to any sense of legal obligation). Not only must the acts concerned amount to a settled practice, but they must also be carried out in such a way as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it.

"Finally, customary international law addresses only those wrongs that are of mutual, and not merely several, concern....

"Customary international law (is) those standards, rules or customs (a) affecting the relationship between states or between an individual and a foreign state, and (b) used by those states for their common good and/or in dealings inter se....

"Even if certain conduct is universally proscribed by States in their domestic law, that fact is not necessarily significant or relevant for purposes of customary international law.... The mere fact that every nation's municipal [i.e., domestic] law may prohibit theft does not incorporate the Eighth Commandment, Thou Shalt Not Steal into the law of nations. It is only where the nations of the world have demonstrated that the wrong is of mutual, and not merely several, concern, by means of express international accords, that a wrong generally recognized becomes an international law violation within the meaning of the statute."


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