Change Without Consent: How Customary International Law Modifies Treaties

Treaties have always had to reconcile two competing interests: the need for enduring agreements and the flexibility to adapt to new circumstances. When treaties were bilateral agreements concluded against static customary international law, it was relatively easy for states to update them as necessary. With the rise of multilateral treaties and swiftly-developing norms, however, treaty text is increasingly running up against conflicting state conduct. Nor are traditional methods of treaty modification - formal amendment, treaty supersession, and adaptive interpretation - able to plausibly resolve all such discrepancies, largely because they require the consent of all state parties. This Article highlights an alternative means of treaty evolution that has been largely ignored in the growing scholarly discourse on this issue: modification by subsequently-developed customary international law. After presenting historical evidence of custom-based alterations that have both lessened and expanded state's international rights and obligations without their explicit consent, this Article proposes a doctrinal explanation for the legitimacy of this approach to treaty modification. Using the controversy over the legality of U.S. humanitarian intervention in Syria as a case study, this Article considers the question of what legal options a state has when it wishes to argue that it is not bound by a particular treaty provision. The usual approach - attempting to reinterpret a treaty's text to permit an action previously understood as forbidden - may actually encourage states to act unilaterally. In contrast, a state that bases its legal argument for the same action on the claim that the treaty has been modified by subsequently-developed customary international law will have to engage in cooperative and consensus-building activity. Somewhat counterintuitively, this non-consensual basis for treaty modification actually requires states to engage in more consensus-respecting conduct.