A Theory of Customary International Law

Abstract

Customary international law ("CIL") is one of two primary forms of international law, the other being the treaty.CIL is typically defined as a "customary practice of states followed from a sense of legal obligation." 3 Conventional wisdom views CIL as a unitary phenomenon that pervades international law and international relations.Governments take care to comply with CIL, and often incorporate its norms into domestic statutes.National courts apply CIL as a rule of decision, or a defense, or a canon of statutory construction.Nations argue about whether certain acts violate CIL.Violations of CIL are grounds for war or an international claim.Legal commentators view CIL to be at the core of the study of international law.And yet CIL remains an enigma. 4It lacks a centralized lawmaker, a centralized executive enforcer, and a centralized, authoritative decision-maker.The content of CIL seems to track the interests of powerful nations.The origin of CIL rules is not understood.We do not know why nations comply with CIL, or even what it means for a nation to comply with CIL.And we lack an explanation for the many changes in CIL rules over time.Both parts of CIL's standard definition raise perennial, and unanswered, questions.It is unclear which state acts count as evidence of a custom, or how broad or consistent state practice must be to satisfy the custom requirement.It is also unclear what it means for a nation to follow a custom from a sense of legal obligation, or how one determines whether such an obligation exists.