Lieber Studies Making and Shaping LOAC Volume – (Un)customary IHL in the 21st Century: Quo Vadis?

Editors’ note: This post is based on the author’s chapter in Making and Shaping the Law of Armed Conflict (Sandesh Sivakumaran and Christian R. Burne eds. 2024), the tenth volume of the Lieber Studies Series published with Oxford University Press.

Considering our 21st-century world offers so many avenues for the expression of explicit consent and considering the indeterminate nature of customary law, it is a matter of some mystery that customary international law remains so widely relied upon for some of the most foundational doctrines of international law. It has had, and continues to have, a profound influence in forming and shaping the substantive rules of many core areas of public international law, with international humanitarian law (IHL) being no exception.

Given that the political climate these days is said to make it impossible to achieve any new treaty law, there are good reasons to expect that the importance of customary international law in the field of IHL will only continue to grow. This makes it important to take a critical look at the state and “health” of customary IHL today, thinking about the way it is developing and what this means (i) for its character as a body of law and (ii) for how it could develop in the future (for reference to “health” of IHL, see Mantilla, p. 172).

In this post, I briefly review the role of customary international law in the field of IHL before focusing on three issues: (i) how IHL has changed the manner in which we think about custom; (ii) how customary international law is developing in a manner that takes it away from its conceptual roots; and (iii) how customary international humanitarian law might develop in the future. All these issues and more are expounded in much more detail in my forthcoming chapter in the book edited by Sandesh Sivakumaran and Christian Burne.

Role of Customary International Law in the Field of IHL

Since the Second World War, the world has witnessed what has been termed a “customary revolution” in the field of IHL (Mantilla, p. 170). The turn to custom started with the case law from the ad hoc tribunals, most prominently the International Criminal Tribunal for the former Yugoslavia (ICTY), which laid out the foundation stones of modern IHL. Their case law established the applicability of many IHL norms to non-international armed conflict as a matter of custom and gave multiple clarifications of how certain substantive terms in customary IHL should be defined and interpreted. Although the ad hoc tribunals have been criticized for having been rather “cavalier” (Darcy, p. 106) in their findings of “custom,” their case law is also widely considered to have been revolutionary, making significant strides in humanizing IHL and clarifying the material, temporal, and personal scope of its substantive provisions.

Indeed, it was likely the existence of this early case law on customary IHL that gave the International Committee of the Red Cross (ICRC) the confidence to proceed with its monumental study on the customary rules of IHL in the 1990s. The study continued the ICTY’s work by taking huge strides in closing the gap between the protections available in international armed conflicts (IAC) and non-international armed conflicts (NIAC), especially when it came to weapons and rules on the conduct of hostilities (Henckaerts). The raison d’etre of the study lay in the perceived insufficiency of the treaty law regime as it applied to both IACs and NIACs.

The study was met with resounding admiration but was also subject to a swathe of critical points as experts picked over aspects of its methodology that related to core issues of customary international law. These criticisms notwithstanding, today most would agree that the study had a tectonic-plate-shifting effect on the landscape of IHL. By extracting the core rules from many of the treaty articles and sub-articles and grouping them into thematically organized sets of “rules,” the customary international law study provided a version of the law of armed conflict that has proved to be temptingly citable to students around the world (oftentimes to their teachers’ frustration!). The study has been relied upon by domestic courts, commissions of inquiries, and the International Criminal Court, and is cited in many military manuals (see Henckaerts & Debuf and Milanovic & Sivakumaran). It also informs the manner in which we think about this body of law, its scope, its making, and its name. This is quite significant when it is realized that certain aspects of the study’s methodology and findings are quirky and certainly not preordained.

IHL Changes the Idea of Custom as a Unitary Body of Law

When seeking to take the temperature of customary IHL (or customary international law more generally) to evaluate its state of health, it is interesting to see that the development of custom in the field of IHL has prompted a number of major shifts in the manner in which the body of law is conceived. For example, the fact that there is a category of customary IHL norms that applies only to NIACs—and another that applies only to IACs—interferes with the traditional idea that customary international law is a body of law that is binding upon all subjects of international law in an identical manner (unless they persistently object) (see Prost, Murray, Fortin for discussions of this). It provides precedent for the idea that different kinds of subjects of international law (e.g., armed groups, States) are bound by different bodies of customary international norms. This is not a matter of concern, but it is important to note because it affects the way in which we think about not only customary international law, but also international legal personality, law-making, and the constituent members of the international legal community.

IHL Changes the Idea That Customary Law Emerges out of Interactions

Modern customary IHL also interferes with the traditional understanding of custom as a body of law emerging out of “a language of interaction” between two or more relevant actors (Fuller, p. 2). Changes in this regard stem from a string of developments that are not necessarily related to one another but end up having a cumulative effect on the legal framework. The first is the increased tendency of international courts and tribunals, particularly in the fields of IHL and human rights law, to adopt a deductive method of custom ascertainment (see e.g., Meron, Kirgis). Deductive methods are characterized by a greater emphasis on opinio juris than State practice, a phenomenon that has been called contra-factual custom. Instead of emphasizing the behaviour of States, it emphasizes principles such as humanity, a supreme common will, and statements of States, thereby reducing the interactional aspect (Simma & Alston).

The second development is the now widely accepted idea that the practice and opinio juris of States can produce legal obligations binding upon a different category of actors than those whose behaviour is being studied, namely non-State armed groups. While it has long been debated whether armed groups should have more of a role in the creation of custom, the current majority position (which includes the view of the ICRC) asserts that this is not necessary (for a review of this debate, see Fortin, p. 348-350; Sassὸli, p. 49-51; Heffes, p. 117-118). While there is surely a justifiable legal argument for this position, it is important to note that the belief that States’ practice and opinio juris can produce an obligation that is binding upon third parties takes understandings of what customary international law is quite far from its conceptual root, i.e., the idea that it emerges out of interactional and reciprocal behaviour by actors connected to the norm. It introduces the idea that customary international law can have a unidirectional dimension, in the sense that dialogical or behavioral relations between one category of actors are capable of producing imperatives for third parties (i.e., armed groups or individuals) (for the term “projecting imperatives,” see Fuller, p. 21).

A third related development can be found in the growing understanding that IHL should not only be seen as a framework of horizontal obligations between the parties to the armed conflict, but also as a set of vertical obligations binding upon the parties to the armed conflict and owed to rights-bearing individuals on the ground, who also have obligations (see e.g. Hill-Cawthorne; Peters; Sassὸli, p. 197-198). These individuals may be prisoners of war or ordinary civilians who are living under the control of one of the parties. Understanding the legal framework as having both horizontal and vertical obligations again throws traditional understandings of customary IHL off balance, because it provides another example of how customary norms are perceived to be binding upon or relevant to a set of actors (i.e., individuals) whose interactional behaviour is not taken into account in its making. Indeed, it is notable that in the traditional custom making process, there is little room for consideration of practice and opinio juris determining the “rights” and “obligations” of individuals and there has so far been little consideration of this question in the scholarship.

Re-Injecting Interactionality Through a Consideration of Rights?

When pondering these developments, it is natural and intriguing to consider whether future iterations of customary international (humanitarian) law might find more space for the concept of “rights.” Indeed, it is interesting to note that in the Asylum case the International Court of Justice indicated that what was required was “constant and uniform usage practiced by the States in question” and that the “usage” should be an “expression of a right appertaining to the State granting asylum and a duty incumbent on the territorial State” (para. 266). The Court’s recognition that State practice may be evidence of both rights and duties is curious and signals an area that warrants further exploration in scholarship.

Notably, a connected discussion has recently taken place with regard to the doctrine of specially affected States. While the traditional view is that States that are “specially affected” are those who bear the most pressing obligations on a particular issue, it has been argued recently that there is room to question this approach. Indeed, Kevin Jon Heller has argued for a wider interpretation of the doctrine of “specially affected States,” so that it includes not only States that engage in a particular practice but also States that are particularly affected by a particular practice. He argues that such a wider interpretation would provide States from the Global South with more power over the formation of custom.

It is relevant that in its customary study, the ICRC took the view that when considering obligations it is necessary to study not only the State whose behaviour is under scrutiny (i.e., the duty bearer) but also the State that would suffer as a result of the obligation’s violation. For example, when considering the rule preventing the export of cultural property from occupied territory, the study takes note of the practice of “States specially affected by occupation” (see here, p. 135). Understanding the “specially affected” test to provide scope for an examination of the State practice and opinio juris ­­of States who are on the receiving end of particular violations and/or have a legal interest in the adherence of particular rulesgoes some way to showing that the customary international law ascertainment process is capable of holding conceptual space to take account of practice relating not only to obligations, but also rights.

A further related development that could provide room for such a conceptual shift in the future lies in the concept of opinio juris communis, which opens the door to considering the interests of civilians, other individuals, or humanity more generally in the customary law making process. First invoked by Oscar Schachter (see here, p. 737-738), this concept was significantly developed by Judge Cançado Trindade during his distinguished career as a judge at the International Court of Justice. In his Dissenting Opinion in Marshall Islands v Pakistan, he accused the Court of having exercised an “inter-state myopia,” such that it has considered the survival of the “hypothetical State . . . rather than that of peoples and individuals, and ultimately of humankind as a whole” (see here, paras. 150 & 165).

Interestingly, I think there is a similarity between the suggestion that Trindade’s opinio juris communis could re-inject an interactionality into the customary framework and a theoretical discussion that took place in the 1990s in human rights scholarship. Here too, scholars were troubled by the fact that although customary norms are supposed to emerge out of State practice and opinio juris, the performance of most human rights obligations “lacks this element of interaction proper [because] . . . it does not ‘run between’ States in any meaningful sense,” but runs between States and individuals (see here, p. 99). Some scholars suggested that the answer lay in an acknowledgment that there are “universal norms of conduct that result from legitimate human expectations and that these expectations are as important a source of law” (Maier). Indeed, Philip Alston famously argued that human rights norms operate at three levels: “as the rights of individuals, as obligations assumed by States, and as legitimate expectations of the international community” (see here, paras. 25-27).

Conclusions

A reader considering these ideas may wonder (i) how such expectations could ever be surveyed and perhaps also (ii) why indeed this question might even matter in the field of customary IHL. While the first question needs much more thought, the answer to the second question is more straightforward. Giving thought to whether there could and should be a greater role for civilians (and arguably other individuals) in the making and shaping of IHL is important because if custom survives at all in the next century, there may be value in making sure it does not become a law of State edicts, rooted in the practice and statements of the powerful. Care must be taken that it remains a body of law based on interactional behaviours, which means taking into account the practice not only of (powerful) States but also less powerful actors who are most at risk of being on the receiving end of military action.

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Katharine Fortin is Associate Professor at the Netherlands Institute of Human Rights, Utrecht University and a researcher at the Montaigne Centre for the Rule of Law and Administration of Justice.

Photo credit: U.S. Army, Pfc. Jaimee Perez