CIL produces a generalized legal order that solves existential threats

This Article challenges that presumption by presenting situations where customary international law has both lessened and expanded states' treaty rights and obligations, thereby supporting the few scholars who have posited - usually in purely theoretical works - that customary international law may modify treaties. By advancing a doctrinal justification for such modification based on lex posterior, this Article also contributes to the growing literature questioning whether the legitimacy of the binding nature of international legal obligations can be grounded solely in state consent. This argument is novel to the extent that it presumes that general - rather than universal - acceptance of a new customary rule may be sufficient to work a treaty modification, thereby avoiding the holdout problem inherent to multilateral treaty modification under the Vienna Convention's consent-focused rules (whereby a single state can upset the consensus of the majority). Finally, this Article advances the counterintuitive argument that this less consensual basis for treaty modification requires a state to engage in more consensus-respecting conduct. When a state wishes to argue against a traditional understanding of a treaty provision, the usual approach of adaptive interpretation - attempting to reinterpret a treaty's text to permit an action previously understood as forbidden - actually encourages states to act unilaterally and risk destabilizing the international legal order. In contrast, a state that bases its legal argument on the claim that the treaty has been modified by subsequently developed customary international law will have to identify and engage in coalition-building conduct. Part I reviews how treaties were historically relatively flexible bilateral agreements concluded against a stable background of default customary international law. While foundational customary norms and bilateral agreements are still the norm, today's international legal structure is complicated by a proliferation in multilateral treaties and an increasing demand for international regulation of new areas, technologies, objects, actions, and ideas. In the absence of directly relevant treaty law, and in need of reliable guiding principles, states are developing practices standardizing their rights and duties in these new spheres. As a result, treaty text is increasingly running up against conflicting state action and swiftly developing customary international law. Part II describes consent-based means of modifying treaties and concludes that these traditional methods do not legitimately resolve all conflicts between treaty and later-in-time customary international law. Part III demonstrates that the possibility of treaty modification by customary international law has long existed in the international legal structure and evaluates different doctrinal justifications for such modification. Part IV employs the threatened U.S. unilateral use of force in Syria as a case study to tease out the relative benefits and drawbacks of these different means of treaty modification. I. A New International Legal Order Customary international law and treaty law are the two primary sources of international legal obligations. Sometimes they operate independently, governing particular fields; sometimes they serve as mutually reinforcing regulations; sometimes one fills the other's lacunae; sometimes they mandate apparently contradictory actions. As this Article is concerned with how customary international may modify treaties, this Part traces how, due to ideological, geopolitical, and technological developments, the relationship between these two sources of international legal obligations has grown more complicated and more prone to conflict. [*242] A. The Classic Account 1. Stable Customary International Law A rule of customary international law is recognized as existing when states generally engage in specific actions (the "state practice" requirement) and accept that those actions are obligatory or permitted (the "opinio juris sive necessitatis" element). Thus, unlike custom in many domestic legal systems, which derives much of its authority from its long-standing nature, customary international law has no formal temporal requirement. Instead, a rule of customary international law is authoritative because states generally abide by it in the belief that it is law. That being said, because of the generalized state practice requirement, customary international law was slow to develop in a world of limited communication and sporadic technological advances. Accordingly, historic customary international law comprised long-established, well-known, and relatively fixed rules governing relations among all states. It regulated the recognition of new states and state responsibilities; the exchange of diplomatic counsels and their immunities; the conduct and resolution of wars; the creation, interpretation, and termination of treaties; and other subjects associated with state interaction. 2. Flexible Treaties Against this background of static customary norms, states concluded [*243] bilateral treaties - written documents memorializing agreements between two states - clarifying their respective legal rights and duties. These treaties were relatively flexible legal regimes: they could be modified or terminated with the consent of states parties, by the conclusion of a subsequent, conflicting treaty between the same parties, by the denunciation of one party after a material breach by the other, or by a fundamental change in circumstances or other supervening event resulting in the impossibility of performing a promised legal obligation. Additionally, certain types of treaties - for example, commercial or trading treaties or treaties of alliance - were generally presumed to allow for unilateral denunciation. Accordingly, treaties have long been celebrated as a source of adaptive positive law that reflects states parties' needs. B. The Modern World Today's international legal structure is far more complicated. Certain customary rules still serve as background defaults governing many areas of state interaction, and the majority of new treaties are still bilateral. But two factors - the rise of multilateral treaties and swiftly developing customary international law - have changed the dynamic between treaty and customary international law, resulting in treaties sometimes being the fixed backdrop against which new state practice and norms develop. 1. Constitutive Treaties The past century has seen a dramatic rise in multilateral treaties - treaties [*244] concluded by multiple countries that often aspire to universal participation. As early as the 1920s, scholars were recognizing that there were "an increasing number of multilateral treaties." According to one study, eighty-six multilateral treaties were concluded in the century between 1648 through 1748 - but more than two thousand such treaties were concluded in the twenty-five years between 1951 and 1975! This proliferation might be traced to a growing conviction that certain global problems - including combating the training and financing of transnational terrorist organizations, minimizing human-driven climate change, and reducing the development or use of weapons