Revisiting Customary IHL

Editors’ note: This post introduces a symposium relating to the ICRC’s Customary International Humanitarian Law Study, featured across Articles of War and Völkerrechtsblog. The symposium 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. 

Since its publication in 2005, the International Committee of the Red Cross (ICRC) Study of Customary International Humanitarian Law has assumed an authoritative position in the identification and application of the law of armed conflict. Consolidating 161 rules drawn from State practice and opinio juris, the Study is the most comprehensive record of the state of customary international humanitarian law (IHL) to date. Its influence is palpable across international tribunals, domestic jurisprudence, military manuals, and academic discourse, even though (persisting) controversy surrounds the Study and its conclusions. For instance, criticism has been raised with regard to an overreliance on written materials, blending opinio juris and State practice (see, e.g., Dinstein, p. 263, 268, 272; Fortin, p. 44-45). Another point of contention concerns the ICRC’s own role as a non-governmental organization in the assessment and influence on customs. Despite these critiques, few would doubt that the Study plays an essential part in revealing some of the red lines actors involved in armed conflicts are not to cross.

Yet considering twenty years have passed, larger updates to the Study might now be due. And given the Zeitgeist casts doubt on the functionality of the law of war, it appears timely to interrogate what makes or breaks its customary rules. Contemporary developments in treaty law illustrate the challenges in reaching international consensus, while also highlighting the increasing relevance of custom. Some States are withdrawing from key treaties governing the use of specific weapons, other actors have challenged the sole authority of States in shaping international (humanitarian) law, and still others criticize the selective application of the law, particularly by Western States. As the debate on a new convention governing the use of lethal autonomous weapons in armed conflict exemplifies, the current state of treaty law faces difficulties in modernized battlefield practices. With no comprehensive treaty on the law of armed conflict on the horizon, opportunities to advance the debate on the state and effectiveness of the law remain scarce.

Renewed international dialogue on custom may help bridge these gaps. The Study’s twentieth anniversary presents a key opportunity to spark a conversation on the customary laws of war and how the rules contained in the Study have been received so far. Whereas State practice collected in the Study is constantly updated by the ICRC and the British Red Cross division at the Lauterpacht Centre for International Law, a new edition of the Study’s rules does not appear to be planned.

Reflections from Scholarship: An Introduction

Bearing in mind how battlefield practice, technology, and legal opinions on custom in IHL may have shifted, the Institute for International Law of Peace and Armed Conflict in Bochum and the Institute for International Peace and Security Law in Cologne have co-convened a workshop to discuss the customary law of armed conflict and the ICRC’s Study. Two decades after its publication, the workshop seeks to place the Study at the center of renewed attention, bringing together junior academics with more senior counterparts to discuss critiques of the Study, and ideas concerning future routes to identify or shape customary IHL.

This joint symposium between Articles of War and Völkerrechtsblog invites readers to engage with the ideas presented over the course of the workshop and those sparked beyond it. The symposium features contributions from scholars who have all approached the Study from different angles. They analyze how the past, present, and future of customary IHL are captured in the Study and inform how a second edition might be shaped.

Their analyses converge on core questions: Who makes customary international (humanitarian) law? What is the Study’s authority, and where are its limits? And, crucially, to what extent does it reflect—or distort—global perspectives on humanitarian law? This introduction delivers an overview of the posts to come.

Tending to Criminal Law in Methodology

Tom Gal interrogates the use of international criminal law (ICL) jurisprudence in identifying customary IHL. She observes that international tribunals such as the International Criminal Tribunals for the former Yugoslavia and Rwanda and the International Criminal Court frequently interpret IHL when defining war crimes, and their judgments are often cited as evidence of customary IHL. Yet, reliance on ICL carries dangers: criminal law is retrospective and individualized, while IHL is prescriptive and State-centered. Using one to build the other risks doctrinal distortion. Gal ultimately recommends a “balanced approach,” by which ICL jurisprudence is treated as a subsidiary source, useful for interpretation but not for identification of custom.

Beyond Headcounts: A Calibrated Approach to Specially Affected States

Stanislau Lashkevich revisits the ICRC’s rejection of the specially affected States doctrine, which has been subject to extensive criticism, most prominently from the United States. He proposes a narrow, criteria-based “calibrated weighting” framework for a few operator-heavy rule clusters. Using two concise case studies—reprisals and environmental protection—his contribution aims to show how targeted weighting can improve descriptive accuracy and legitimacy without conferring a veto on any State.

A Study 2.0 Featuring the Martens Clause: Tool Rather Than Subject

Paulina Rob discusses proposals to codify the Martens clause in a new edition of the Study. Looking at different interpretations of the clause, Rob illustrates that its character as a secondary norm militates against its inclusion in the Study; a codification project of primary norms. Alternatively, she considers a potential role for the Martens clause in substantiating the methodology of a new edition of the Study.

The Study’s Value in Researching Legal History

Nadia Kornioti contends that the Study’s significance far exceeds its utility in legal practice. She argues that while the Study was primarily designed as a tool for policymakers, courts, and humanitarian actors, it has been undervalued as an academic resource with significant encyclopaedic impact on research uncovering legal history. The Study pinpoints the development of the law over time, containing “dual-nature documents” that make it simultaneously a legal tool and a historical source. Therefore, the Study offers a bridge between doctrine and its formative context.

Customary Law in the Negative: Revealing Unsettled Norms

Mina Radončić proposes that, alongside codifying existing rules, the ICRC should also signal areas where the law remains unsettled. Her case study is the nationality requirement for prisoner of war (POW) status: must a State grant POW protection to its own nationals captured while fighting for the enemy? Radončić’s intervention is twofold. First, she stresses that mapping unsettled norms would help prevent States from exploiting ambiguity, for example, to justify mistreatment of captured fighters. Second, she proposes criteria for inclusion: the ICRC should flag unsettled norms where claims of custom risk less protective outcomes, and where clarification would fill gaps in treaty law. In essence, Radončić calls for a “non finito” approach, acknowledging incompleteness as an asset rather than a weakness.

The Limits of Creation: Non-Binding Norms and Progressive Development

Ilya Ivanov examines how the Study has, in effect, created new rules or modified existing ones, despite its claim to constitute a mere codification project, documenting lex lata. Ivanov identifies three categories of such innovations. He places his focus on rules inspired by international human rights law and extensions of norms from international to non-international armed conflict.

Getting It Right? Customary IHL Identification

Vincent Widdig argues that, while the Study remains a valuable baseline, its methodology for identifying customary IHL requires both a deepening and a widening of the scope in view of modern warfare. Heavy reliance on assertions made in manuals and UN statements blurred the lines between treaty interpretation and custom, while anglophone biases and an understatement of real-world practice and opinio juris may further have distorted the picture. Widdig calls for a rebalancing toward operational evidence, patterning silence, a clearer test for opinio juris, systematic inclusion of smaller-State practice, and bringing non-State armed groups as well as new theatres of war (such as cyber or urban domains) into view.

The Perils of Issuing a Second Edition of the Study

Linus Mührel reflects on the question whether the ICRC should publish a new edition of its Customary International Humanitarian Law Study. He argues that while the Study has long enjoyed exceptional authority in legal practice and academia, a revision today could carry serious risks: depending on its methodology and content, a revised Study might attract strong criticism and thereby weaken not only the authority of the Study itself but also the standing of IHL more broadly.

A Pluralist Future of Customary International Humanitarian Law

Taken together, the ideas introduced in this series highlight the contested terrain of customary international humanitarian law two decades after the ICRC’s Study was first published. These perspectives paint a picture of a field in flux: methodologically contested, normatively ambitious, and geopolitically imbalanced. Yet they also reveal opportunities. Recognizing unsettled norms, integrating non-State and Global South practices, distinguishing identification from interpretation, and using the Study’s encyclopaedic value could contribute towards the support of legitimate, inclusive, and effective custom.

The Study at twenty is less a final word than an invitation to States, scholars, practitioners, and armed groups alike to shape the future of IHL. With these ideas, we hope to set the stage for fruitful discussion.