Revisiting Customary IHL Series – A Question of Methodology: Should International Criminal Law Shape IHL?
Editors’ note: This post is part of a series relating to the ICRC’s Customary International Humanitarian Law Study, featured across Articles of War and Völkerrechtsblog. The introductory post is available here. The series highlights presentations delivered at the young researchers’ workshop, Customary IHL: Revisiting the ICRC’s Study at 20, hosted by the Institute for International Peace and Security Law (University of Cologne) and the Institute for International Law of Peace and Armed Conflict (Ruhr University Bochum) on September 18-19, 2025.
As wars rage on across the globe, legal debates over the rules of armed conflict are more than academic. Scholars typically refer to international humanitarian law (IHL), which sets the framework for how wars are fought, and international criminal law (ICL), the legal tool to enforce accountability when those rules are broken. Increasingly, however, courts and commentators look to ICL jurisprudence itself as evidence of customary IHL. That trend raises a pressing methodological question: should ICL, or more specifically, international criminal jurisprudence, shape the rules that bind States in war?
This post reflects on that question, drawing on critiques of the International Committee of the Red Cross (ICRC) Customary IHL Study and the role of international criminal jurisprudence in shaping perceptions of custom. The argument here is not that ICL jurisprudence is irrelevant. Rather, it is that while criminal judgments may illuminate how rules are interpreted, they cannot substitute for the State practice and opinio juris that lie at the core of customary law.
Identification Versus Interpretation of Customary Rules
International law distinguishes between identifying a customary rule and interpreting it. The International Law Commission’s Draft Conclusions on Customary International Law (2018) reaffirmed the orthodox view: a customary norm requires both general State practice and a belief that such practice is legally obligatory (opinio juris). States remain the principal actors in this process. Yet international criminal tribunals often blur the line. Instead of painstakingly proving a rule’s existence through evidence of State conduct, judgments sometimes assert custom and then move quickly to interpreting its content. As Anthea Roberts noted in her classic piece on traditional and modern approaches to custom, this exercise risks substituting judicial aspiration for demonstrable practice. At best, criminal jurisprudence should serve as a subsidiary source, a tool for interpretation rather than identification.
Determining customary IHL is a uniquely challenging task. Unlike treaties, which bind only signatories, customary IHL applies to all States unless they persistently object. That universality is crucial in a field where treaty-making has stagnated. Core IHL principles such as distinction, proportionality, and the prohibition of unnecessary suffering are customary in nature. But the process of proving custom in war is notoriously difficult. Belligerents often hide or downplay their practice. Violations may reflect rogue individuals rather than State policy. Military manuals blur law and operational policy. Treaties raise the “Baxter paradox”: if States comply with a norm, they may do so because it is a treaty obligation, not custom; if they breach it, it counts against custom. Against this backdrop, it is tempting to lean heavily on international judgments.
The turn to international proceedings is evident in the ICRC Study, which remains the most influential mapping of customary IHL rules to date. The Study has shaped scholarship, military manuals, and even domestic law. Yet its methodology has drawn sharp critique. The United States objected from the outset, warning that the Study relied too heavily on soft-law sources and blurred State practice with opinio juris. Scholars have noted the Study’s tendency to extend rules designed for international armed conflicts into non-international contexts without sufficient evidence (see Sassòli, p. 52-53). More generally, critics argue that the Study sometimes treats criminal judgments as quasi-State practice, overstating their evidentiary value. Despite these concerns, the Study has become a touchstone, which makes scrutiny of its reliance on ICL jurisprudence all the more urgent.
A Black or White View: It’s Either Lawful Conduct or a War Crime
The appeal of criminal judgments is clear. Before the Rome Statute, written ICL was sparse. Tribunals like Nuremberg and Tokyo relied heavily on custom to establish jurisdiction and comply with the principle of legality (nullum crimen sine lege) (see Cassese’s 2013 review, p. 13-14). The International Criminal Tribunal for the former Yugoslavia’s (ICTY) Tadić decision provided much-needed clarity by defining armed conflicts and extending certain international armed conflict rules to non-international armed conflicts (paras. 70-73, 96-97). ICL also strengthens compliance, reinforcing deterrence and encouraging States to incorporate IHL into domestic law. In these ways, ICL has undeniably advanced the content and reach of IHL.
But equating criminal judgments with State practice carries dangers. Courts sometimes appear to “make law,” as in Tadić or in expansive definitions of liability. While Professor Meron has defended such judicial creativity as preserving the “humanity” of the law, Professors Roberts and Blum have both warned that it risks undermining legitimacy. Seeing IHL through the lens of criminal trials risks reducing it to “war crimes or lawful conduct,” ignoring the many protective rules not criminalized (Sassòli, p. 485-86).
Additionally, different chambers of the same tribunal have contradicted each other, for example, with respect to conflict classification and the required elements therefor (see the Tadić classification changes between the Trial Chamber and Appeals Chamber, and the Haradinaj judgment, with respect to the “intensity” or “protracted” element of NIAC). As Professor Sassòli has cautioned, addressing IHL through the prism of ICL may ultimately weaken compliance, and may explain States’ reluctance to approach other IHL compliance mechanisms (Sassòli, p. 485-487; see also Blum).
Perhaps the most striking example is the treatment of rape as a war crime. In Akayesu the International Criminal Tribunal for Rwanda issued a landmark definition of rape in international law, tying it to violations of personal dignity under Common Article 3 to the Geneva Conventions. The judgment was groundbreaking for gender justice, yet methodologically thin. It cited no State practice and blurred crimes against humanity with war crimes. The Study later listed Akayesu, along with Furundžija and Kunarac, as evidence of the customary prohibition of rape in war. Yet critics note that this imports a definition forged in crimes against humanity into the war crimes context, without demonstrating that States had accepted it as law.
The International Criminal Court (ICC) later convicted commanders for rapes committed even within their own armed groups, including against child soldiers (Ntaganda and Ongwen). These judgments powerfully advanced accountability for an appalling conduct that harmed child soldiers forcibly recruited by armed groups in this context. However, they arguably stretched IHL’s scope by importing human rights concepts, blurring the distinction between war crimes, traditionally committed by the belligerent toward the adverse party to the conflict, as well as the definition of active participation in hostilities. When the ICRC incorporated this line into its revised Commentary on Common Article 3 (para. 547, n. 293), it risked embedding this advancement of the law, not reflected in State practice, as part of IHL definitions, rules, and eventually custom.
It almost goes without saying: the dilemma is not whether rape and sexual violence are prohibited. They clearly are. The real question is how to establish the rule’s customary status. Do we rely on scattered State practice and opinio juris, or shortcut the process by invoking tribunal judgments? The latter approach may be expedient, but it risks undermining methodological discipline and, ultimately, the legitimacy of both IHL and ICL.
Returning to Meaningful Interpretation: A Balanced Approach
What then is the role of ICL jurisprudence in shaping customary IHL? A balanced approach would recognize that courts and tribunals can illuminate the interpretation of IHL but should not be treated as State practice themselves. Methodological clarity is essential: when the Study or scholars cite judgments, they should distinguish whether these serve as evidence of State practice, opinio juris, or merely persuasive interpretation. Respecting the distinctions between IHL and ICL matters. Conflating them risks raising thresholds unrealistically or erasing important rules that are not criminalized. Where judgments do fill gaps, they should be corroborated with State practice to avoid circular reasoning. Ultimately, careful assessment of international judgments and their legal weight with respect to particular conduct is indispensable to maintaining the legitimacy of customary IHL.
International criminal law has transformed the enforcement of humanitarian law, from Nuremberg to the ICC. It has clarified vague norms, expanded protections, and strengthened compliance. Yet its jurisprudence cannot replace the centrality of State practice in identifying customary rules.
As we look to the future development of customary IHL, the temptation to shortcut through tribunal judgments must be resisted. Accountability is vital, but so is methodological rigor. The legitimacy of both IHL and ICL depends on remembering their different foundations: one rooted in State conduct, the other in individual responsibility. Only by respecting the distinction between both bodies of law can we ensure that IHL remains coherent, credible, and, above all, protective of those who would otherwise suffer the consequences of its breach.
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Dr Tom Gal is a Lecturer at the Reichman University, Herzeliya, Israel.
The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.
Photo credit: Giammarco Boscaro via Unsplash
