Customary International Law

Customary international law is one of the two primary sources of international law alongside treaties, with general principles of law and judicial decisions as subsidiary sources. It develops through the accumulation of state practice — what governments actually do, evidenced by legislation, executive decisions, military manuals, diplomatic acts, and judicial decisions — combined with opinio juris, the belief among states that the practice is legally required rather than merely habitual or politically motivated. The International Court of Justice required both elements in the North Sea Continental Shelf Cases (1969): consistent practice alone is insufficient; states must also believe the rule is obligatory. This two-element requirement prevents diplomatic norms and political conventions from automatically hardening into binding law.

CIL binds all states, including those that have not ratified relevant treaties. A state that refuses to ratify the Chemical Weapons Convention or the Geneva Conventions is still bound by the equivalent customary international law rules that predate or parallel those treaties. The persistent objector doctrine creates a narrow exception: a state that consistently and openly objects to a developing customary rule during its formation is not bound — but this requires timely, consistent, and publicly stated objection that almost no state successfully maintains against established norms. Major powers have invoked the persistent objector doctrine occasionally, but international tribunals have rarely accepted the argument as applied to well-established rules.

The relationship between CIL and US domestic law is governed by the Paquete Habana doctrine (1900), in which the Supreme Court held that customary international law 'is part of our law' and courts must apply it. However, a later act of Congress can override CIL under the 'later-in-time' rule — domestic statute prevails over conflicting CIL when Congress has clearly spoken. This creates a two-track analysis: CIL binds the US as a matter of international obligation, but US courts may be required to apply domestic law even where it conflicts, if Congress has expressly mandated the conflicting outcome. This tension between international obligation and domestic statutory supremacy is a persistent feature of US foreign relations law and is central to debates over torture, targeted killing, and detention practices.