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Customary International Law

The Transformation of State Practice and Opinio Juris into Customary International Law: Understanding How Custom Acquires Normative Force

Mollie Bailey Hammerton

International cooperation between states is becoming increasingly difficult due to deepening geopolitical “tensions” and “divides.” Guzman and Hsiang have suggested that customary international law (CIL) may have the “potential” to resolve such “intractable cooperation” issues because CIL is binding on all states and “consent” is not required for it “to be binding.” However, the normative force of CIL and how it becomes binding upon states remains unsettled in international law. In this respect, this article seeks to clarify how CIL becomes binding upon states by examining the two elements of customary international law: state practice and opinio juris. It argues that the Voluntarist understanding of CIL places disproportionate emphasis on the requirement of consent and that the normative force of CIL depends on there being a general practice that states generally accept as law. In this regard, this article analyses the persistent objector rule and argues that it provides an exception to the universal binding force of CIL by allowing states to unilaterally express their will. Lastly, it is suggested that although the two-elements of CIL apply more strictly for particular CIL, particular CIL should not be deemed a “tacit agreement.” Thus, for general CIL to acquire normative force and become binding on all states there must be a general practice that states accept as law.

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