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The Yale Law Journal Online


Although customary international law (CIL) has historically been one of the principal forms of international law, it is plagued by debates and uncertainties about its proper sources, its content, its usefulness, and its normative attractiveness.1 While some of these debates and uncertainties are longstanding, they have intensified in recent years, in part because of the rise of multilateral treaty-making, which allows nations collectively to negotiate and codify broad areas of international law instead of relying on unwritten custom. Moreover, it has become increasingly apparent that CIL is structurally unable to address many of the world’s most pressing problems, such as (to name a few) nuclear proliferation, global warming, and international financial stability. Perhaps most fundamentally, there “is no coherent or agreed upon theory to justify [CIL’s] role or explain its doctrine.”2 For these and other reasons, CIL is “under attack from all sides.”3

In light of the current state of CIL, it is worth thinking creatively about how to improve this body of international law. One difference between the way that many treaties are structured and the way that CIL is structured concerns withdrawal rights. The conventional wisdom among international law scholars is that, once a rule of CIL becomes established, no nation has the legal right to withdraw from the rule. Instead, if a nation wants to change a rule of CIL, either it must convince other nations to enter into a treaty overriding the rule, or it must violate the rule and hope that other nations will acquiesce to the violation. This regime applies even to nations that enter the international system after a CIL rule is formed. In a recent article, Withdrawing from International Custom, we referred to this conventional wisdom as the “Mandatory View” of CIL.4

In contrast to the Mandatory View, many treaties, even those that address fundamental issues of international public policy, expressly allow for unilateral withdrawal, sometimes conditioned on a notice period.5 In addition, other treaties are thought to allow for withdrawal by implication—as a function, for example, of their subject matter. Furthermore, even when a treaty does not expressly or implicitly allow for withdrawal, a nation may have the legal right to terminate the treaty as a result of developments such as a material breach of the treaty by another party or a fundamental change of circumstances. We suggested in Withdrawing that the contrast between the extensive and variable exit rights under treaties and the purported lack of any exit rights under CIL was puzzling and deserved study, especially given the overlap that exists today between the content of treaties and of CIL and the frequent reference to treaties as evidence of CIL.6

We then turned in Withdrawing to a consideration of history. The Mandatory View, we explained, was not always the canonical understanding of CIL. Some of the classic publicists on international law, such as Emmerich de Vattel, thought that nations had the legal right to withdraw from some rules of international custom, a proposition that we referred to as the “Default View” of CIL.7 Also, a number of the U.S. Supreme Court’s early international law decisions assumed such a Default View.8 We attempted to discern when the intellectual shift to the Mandatory View occurred, and we suggested that the shift—at least in the international law treatises—may have taken place most sharply in the late nineteenth and early twentieth centuries.9 We also found indications that the shift may have been related in part to efforts to bind new states to customary rules that had been established by Western powers.10

We did not argue that this history dictated any particular conclusions about how CIL should operate today. Instead, we noted that it “shows that the Mandatory View is not the only possible approach to CIL, and that an international legal system could potentially operate despite the allowance of some CIL withdrawal rights.”11 The remainder of Withdrawing was dedicated to an examination of the functional desirability of the Mandatory View. We found little explanation for the Mandatory View in the literature and thus were compelled to speculate about its justifications. Drawing upon scholarship in the areas of contract theory, corporate law, voting rights, and constitutional design, we concluded that reasonable arguments could be made for disallowing exit with respect to those areas of CIL that address significant interstate externalities or agency problems but that it was difficult to justify a disallowance of opt-out rights across the board for all of CIL.12

Finally, we suggested that allowing for withdrawal rights under CIL could help revitalize this body of international law.13 Withdrawal rights would make it easier for states to innovate in addressing modern problems because the additional flexibility provided by such rights would lower the cost of creating new rules of CIL as well as the cost of pushing for alterations to existing rules. In addition, the invocation of withdrawal rights would help provide information to the international system about the position of nations on both the content of particular CIL rules and their level of commitment to them. Furthermore, providing a lawful avenue for exiting from CIL rules in the face of changed circumstances would increase the likelihood of collective enforcement of such rules against states that have not exercised the right of withdrawal since the line between breach and exit would be more sharply delineated.

Withdrawing challenges conventional wisdom, so it is not surprising that it has generated strong reactions. The scholars who have written responses in The Yale Law Journal Online—Lea Brilmayer, Bill Dodge, David Luban, Isaias Tesfalidet, and Carlos Vázquez—critique our analysis on a number of grounds. In this Essay, we attempt to address these criticisms, while also highlighting areas of research that might help move the debate forward. Throughout the Essay, we emphasize a recurring theme, which is the need to improve and revitalize CIL. Before proceeding, we want to express our gratitude to the respondents for engaging with our project and to The Yale Law Journal for facilitating this dialogue.14

In Part I, we discuss the endorsement of the Default View by some of the classic international law publicists, including Vattel. In Part II, we consider the shift in the international law treatises to a Mandatory View of CIL and the potential connections between that shift and colonialism. In Part III, we revisit the modern disparity in withdrawal rights between treaties and CIL. Finally, in Part IV, we address a number of functional considerations that are relevant in choosing between the Default and Mandatory Views.

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