Curtis Bradley's new edited book, Custom's Future: International Law in a Changing World, boldly tackles the questions at the forefront of the contemporary debate about customary international law (CIL): what constitutes state practice; how much state practice is enough; how much time is required; what demonstrates a sense of legal obligation; what is the extent to which treaties and soft law documents serve as evidence of customary law; what is the role that nonstate actors play in generating or confirming rules of customary law; and what does the future hold for CIL?

Custom's Future has all the hallmarks of a highly influential collective work. The topic is extremely timely and of great theoretical and practical significance, and the authors are among the leading scholars in the field, with expertise in a variety of specialty areas, who bring creative and original thinking to bear on their contributions. As someone who recently published a book with Cambridge University Press on this topic,Footnote ¹ this reviewer was surprised at the number of fresh insights and the amount of new historic facts I picked up from the writings of Bradley, a professor of law and public policy studies at Duke University, and his contributing authors.

As the concept of CIL predates the creation of the modern state system, one might be tempted to conclude that there is not much more of import that could be written on this subject. But continuing and new debates about the formation and content of CIL prompted the UN's International Law Commission (ILC) to take up the subject for a multiyear project in 2011. Bradley is to be lauded for including a chapter in his book coauthored by Michael Wood, the ILC's Special Rapporteur on the topic of CIL. At the same time that Bradley's book was going to press, Wood and his colleagues at the ILC were promulgating a set of conclusions with commentaries about CIL. The initial sixteen draft conclusions/commentaries were issued in 2016, along with an announcement that the ILC would publish a revised final text in 2018 based on the reactions of states and experts.Footnote ²

In this chapter, Wood and his coauthor Omri Sender remind us that CIL “has withstood the test of time, as well as significant political and doctrinal challenges, and remains both useful and credible” (p. 369). They write that the ILC's current work on the subject has verified that “the once-fashionable notion that custom is in crisis, in decline, or even ‘dead,’ was never much more than an academic (or political) conceit; it had no basis in reality, no support among states, and no following among practitioners” (pp. 367–68).

Bradley includes among his chapter contributors some who are highly critical of CIL and argue that CIL has outlived its usefulness,Footnote ³ but the majority of the chapters attempt to assist the international community to better understand and apply CIL, not argue for its demise. Before examining some of the chapter authors’ proposals, it is helpful to briefly examine custom's contemporary significance.

Judge Theodor Meron, the President of the International Criminal Tribunal for the Former Yugoslavia, recently wrote that “[c]ustomary international law now comes up in almost every international court and tribunal, in almost every case, and frequently has an impact on the outcome.”Footnote ⁴ Notwithstanding extensive codification over the past seventy years, there are four ways that CIL continues to have vitality.

First, CIL still plays a significant role in uncodified areas of international law. For example, departing from the holdings of the International Court of Justice (ICJ) in Nicaragua v. United States and Democratic Republic of Congo v. Belgium, the UN Security Council unanimously confirmed in November 2015Footnote ⁵ that the United States and its allies had a CIL right of self-defense to attack ISIS targets in Syria where the government of Syria was unable or unwilling to quash the threat to Iraq and the international community posed by the nonstate actor.Footnote ⁶

Second, in some ways, CIL possesses more jurisprudential power than does treaty law—meaning that it can have a more significant effect upon international behavior.Footnote ⁷ For example, unlike treaties, which bind only parties, CIL binds all states, save those who persistently objected during its formation.Footnote ⁸ Some international law rules coexist in treaties and custom, and thus CIL expands the reach of the rules to states that have not yet ratified the treaty; further the CIL status of the rules can apply to actions of treaty parties that predated the entry into force of the treaty.Footnote ⁹ Moreover, unlike treaties which permit withdrawal simply by giving advance notice, “customary international law does not recognize a unilateral right” to escape its obligations.Footnote ¹⁰

Third, while conventional wisdom suggests that treaties are more precise than customary rules, the reverse is sometimes the case. That is because multilateral treaties may be filled with intentional ambiguity to facilitate diplomatic agreement among parties with diverse interests, whereas custom is often derived from decisions on specific questions in concrete cases.Footnote ¹¹

And fourth, while the traditional view is that customary norms take decades or even centuries to ripen into law, the reality is that CIL may in some cases form at a faster pace than the negotiation and entry into force of multilateral treaties.Footnote ¹² This is especially true with respect to fundamental technological, environmental, cultural, and humanitarian developments.Footnote ¹³

A rule of CIL is usually deemed to exist when, in the words of the ICJ Statute, there is “a general practice accepted as law.”Footnote ¹⁴ This traditional definition has two elements: (1) widespread state practice that is (2) followed out of a sense of legal obligation (referred to as opinio juris). Bradley and several of the book's authors find troubling the so-called “chronological paradox” that the traditional definition of CIL raises (p. 2). They ask how can a new rule of CIL emerge if custom requires a sense of legal obligation from the start? In other words, they suggest that the first state to invoke a new rule of CIL cannot seriously entertain the view that it is acting in accordance with the law. Rather, as a custom pioneer, such a state is self-consciously seeking to change the law. In accord with this criticism, Judge Lachs famously observed in the North Sea Continental Shelf cases that to require a conviction that the conduct is already a matter of legal obligation is to deny the possibility of developing new rules of customary law.Footnote ¹⁵

A number of the book's authors take issue with the conventional answer to this quandary, namely that the custom pioneers are simply acting in the mistaken belief that the new practice reflects existing law. Sharing their skepticism of the mistaken belief riposte, this reviewer has written that it is more likely in such a case that the pioneers of the customary rule believe it would be desirable if the preferred rule were the law so they purposely “couch their innovation in the language of existing law, even when they know they are actually breaking new ground.”Footnote ¹⁶

Desiring to improve the definition of CIL to avoid this paradox, several of the book's chapter authors advocate new formulations and approaches. For example, Bradley argues in his chapter, “Customary International Law Adjudication as Common Law Adjudication,” for a more “forward-looking and progressive” approach than the standard two-element formulation, contending that “adjudicators look to past practice but necessarily make choices about how to describe it, which baselines to apply in evaluating it, and whether and when to extend it to new situations” (pp. 5, 34). Brian Lepard, in turn, contends in his chapter “Customary International Law as a Dynamic Process,” that CIL should not be conceived as a static form of law “embedded” in international practice, but rather “as a dynamic method of lawmaking” (pp. 5, 63–64). He proposes that CIL should be reconceptualized “as a belief by states generally that it is desirable now or in the near future to have an authoritative legal principle or norm prescribing, permitting, or prohibiting certain conduct, apart from treaty obligations” (pp. 5, 63). And John Tasioulas, in a chapter titled “Custom, Jus Cogens, and Human Rights,” argues that “opinio juris involves the judgment that a norm is already part of customary international law and that (compliance with) it is morally justified …; or that, as a moral matter, it should be established as law through the process of general state practice and opinion juris …; or else some mixture of these two attitudes” (p. 97).

Referencing the work of Professor Maurice Mendelson,Footnote ¹⁷ I have written that the paradox can be solved by understanding that the ICJ's formulation—”accepted as law”—can have two concurrent meanings. At the early formation stage, “acceptance” means consent to an emerging rule, and in the later stage “acceptance” means acknowledgment that the rule has gained the force of law.Footnote ¹⁸ The advantage of this view is that it does not require the international community to embrace a brand-new formulation. Where the authors of Custom's Future are critical of the ambiguity surrounding the negotiating record and wording of the ICJ Statute, Mendelson avows that the ambiguity actually represents a simple and elegant solution to the paradox.

Other chapters of Custom's Future make an important contribution by providing empirical data and analysis about how courts and tribunals recognize and apply CIL in a variety of contexts. The chapter by Stephen Choi and Mitu Gulati, titled “Customary International Law: How Do Courts Do It?,” for example, establishes that three kinds of evidence that are aspirational in nature in fact play a large role in CIL determinations, namely “UN resolutions, other UN material (committee reports, conference reports, etc.), and domestic statutes” (p. 133). Similarly, Monica Hakimi's chapter, “Custom's Method and Process: Lessons from Humanitarian Law,” discusses evidence indicating that “nonstate actors who are charged with finding CIL can be extremely influential in making CIL” (p. 163). In a chapter titled “The Strange Vitality of Custom in the International Protection of Contracts, Property, and Commerce,” Chin Leng Lim documents CIL's importance in international economic law, particularly in the interpretation of investment treaty clauses. And Larissa van den Herik's chapter, “The Decline of Customary International Law as a Source of International Criminal Law,” examines the impact of CIL on the jurisprudence of international criminal tribunals.

The final chapters, written by Niels Petson, Andrew Guzman, Jerome Hsiang, Laurence Helfer, Timothy Meyer, Jan Wouters, Linda Mamid, and culminating with Omri Sender's and Michael Wood's concluding chapter, “Custom's Bright Future: The Continuing Importance of Customary International Law,” examine various theories that explain the continuing relevance of CIL and discuss the outlook for CIL's future. Michael Wood tells us that the original title for Bradly's book was “Custom in Crisis: International Law in a Changing World,” but these chapters and the general tenor of the book are more optimistic and Bradley wisely went with a more neutral title.

At the end of his introduction, Bradley writes that “this book aims to make its own contribution to custom's future” (p. 10). Given the timing of its publication, the credentials of its chapter authors, and the useful empirical data and insightful analysis it contains, Bradley's book will undoubtedly influence the content of the ILC's exposition on the formation and content of CIL, as well as serve as the touchstone for the continuing contemporary debate on this subject. Without overstatement, I can recommend Custom's Future as essential reading for anyone practicing or writing in the field of international law.