Saving Customary International Law

The theory of international law has a variety of problems, but none are more glaring than those present in customary international law (CIL). Though CIL is one of the two main sources of international law, it is both poorly understood and under-theorized. Attracted by this weakness, critics have sought to demonstrate that CIL is irrelevant or non-existent. This Article lays a solid theoretical foundation for CIL and responds to critics by showing that CIL can be a meaningful influence on state behavior. Consistent with much of the current writing on international law, including the strongest criticisms in both legal and political science scholarship, the Article makes standard rational choice assumptions about state behavior. It is assumed that states are self-interested and that they will comply with international law only when it is in their interest to do so. It is shown how, under these assumptions CIL can exist and can influence state behavior. What emerges is a robust and coherent theory of CIL. The theory is then used to examine the doctrine of CIL. Where existing views of CIL are consistent with the theory, the Article provides them with a stronger and more satisfying theoretical justification. For example, the theory provides a novel and more persuasive explanation for the persistent objector doctrine. Where existing perspectives are in conflict with the theoretical approach, the Article explains how and why current views should be adapted. For example, the Article argues that opinio juris should be at the center of our understanding of CIL, and that state practice, traditionally considered the second requirement for the establishment of a rule of CIL, should be viewed only as evidence of opinio juris rather than an independent requirement. In addition to the opinio juris and practice requirements, the article examines the most salient doctrinal issues in CIL, including the relationship between treaties and CIL formation, the persistent and subsequent objector doctrines, the role of new states, instant custom, regional or special custom, and the relationship between jus cogens norms and CIL.