Identifying Customary LOAC in Practice

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The following post is part of a symposium based on a conference panel that discussed issues of customary law of armed conflict, at the 4th Israel Defense Forces (IDF) Military Advocate General (MAG) Conference on the Law of Armed Conflict, held in Herzliya, Israel, during May 8-10, 2023. The post is based on Colonel Katzir’s presentation in the panel.

In a post on this Blog a few years ago, Jorge Viñuales observed that “generalist international lawyers have become an endangered species.” This could be seen as an expected result of the ever-growing strive among many international law scholars for expertise in specialized niches. As law of armed conflict (LOAC) practitioners, a question we would like to raise premised upon Viñuales’ observation is – are LOAC experts sufficiently sensitive to generalist issues of international law? Specifically, we would like to focus in this post on the cardinal issue of customary international law (CIL) identification.

In advising the armed forces of a State, identifying and applying rules of CIL in the field of LOAC is a daily routine. Israel, like several other states, is not a party to the First Additional Protocol to the Geneva Conventions (API) and therefore CIL is a primary source of law we turn to when providing legal counsel on many issues in LOAC – particularly regarding the conduct of hostilities. Yet, a recurring experience in our work with LOAC academic literature is coming across statements on the purported customary status of rules that lack adequate substantiation or a recognizable methodology. We, along with our colleagues, often feel the need to be self-reliant in conducting inductive research – collecting and analyzing state practice and opinio juris – in order to assess the existence and content of customary rules.

Drawing from our personal experiences, this post will present three main observations, addressing crucial methodological aspects related to the identification of CIL that we often encounter while practicing LOAC.

Theory and Practice

The well-accepted method of identifying customary law is the “two-element approach,” requiring the creation of a rule a general practice of states accepted as law (opinio juris). In practice, identifying state practice and opinio juris in the LOAC context involves arduous work that often requires consideration of certain factors:

1. When examining both states’ practice in the field and their statements, it is important to discern whether they stem from a sense of a legal obligation. Consider the example of weapons reviews (a subject providing an exceptional example of a successful implementation of the two-element approach in a recent academic LOAC study). State parties to API that conduct legal reviews of new weapons logically do so to fulfill their obligation under Article 36 of API. Determining whether they also perceive this as a customary obligation is a separate matter altogether. As the ILC noted, “[s]eeking to comply with a treaty obligation… is not acceptance as law for the purposes of identifying customary international law… this [intent] would need to be proved.” States not party to API which conduct such reviews might hypothetically see it as a customary obligation – or they may do so for other reasons, such as compliance with domestic legislation, or exercising good policy (as is the case with Israel). Bearing in mind the different motivations a state may have for its conduct, only if there is clear and positive evidence it is done out of a sense of legal obligation to a customary norm, should the conduct “count” towards the creation of customary law. Looking for a state’s motivation and seeking whether such evidence exists must be an integral part of any CIL research.

2. Another distinct factor that warrants consideration is the limited number of states participating in armed conflicts. When identifying customary rules, particular weight is given to the conduct of “specially affected” states – in this case, states engaged in armed conflict – as their practice is essential in the formation of customary law. As Judge Tomka recently observed, “[t]he generality of the practice is not necessarily to be ascertained with reference to the totality of states, but rather with reference to the states that are concerned with, or in a position to contribute to the formation of, the putative rule.” Without being able to point to state practice and opinio juris of the specially affected states, it is difficult to maintain that a customary rule has formed.

3. It is also critical to properly assess the evidentiary value of states’ inaction in supposing the existence of a customary rule. For inaction to constitute state practice, it must be shown to involve a conscious and deliberate choice. Only then is it reasonable to proceed to the stage of assessing opinio juris, and just like with action, it is necessary to thoroughly investigate whether the particular inaction was motivated by a sense of legal obligation to refrain from action – meaning a recognition of a legal prohibition – and not due to some other factor. Consequently, utmost caution is necessary when assessing the customary character of LOAC rules based on states’ inaction or silence. One of many examples illustrating this point well comes from prize law (an area in which Israel has recent experience): Inferring that states refraining to exercise certain rights under prize law constitutes a change in the status of centuries-old law, is unfounded – as the vast majority of recent armed conflicts have not involved naval warfare of a significant scale.

4. General practice observed among many states on a certain matter can serve as evidence that the practice in question is not prohibited by customary law, based on the assumption that states generally act in good faith. For instance, the use of area weapons and land-denial weapons have been – and continue to be – longstanding military practices shared by militaries universally, notwithstanding questions that they are said to raise in view of targeting law. Such widespread usage provides strong indications that states generally perceive such weapons as lawful in principle.

Is contemporary LOAC scholarship sufficiently sensitive to these considerations? In our view, in too many cases the answer is – not enough.

In fact, many academic publications do not attempt to assess practice and opinio juris but simply echo previous academic publications, without critically reviewing the original analysis they rely upon. Central among the cited sources is, of course, the ICRC Customary IHL Study. While the Study is undoubtedly a seminal project – and a very useful one too – we argue that caution is required when utilizing this study, necessitating meticulous scrutiny of its methodology in each case.

The Study’s methodology – and part of its conclusions – has been subject to significant criticism, including for the reasons that it often relies on limited evidence to draw universal conclusions or does not give proper weight to the practice of states involved in armed conflicts; it combines the examination of state practice and opinio juris without adequately considering each separately; and it draws certain conclusions on the content of the rule using elements of interpretation which diverge from the two-element approach (we’ll return to this point in the last section of this post).

Another criticism regarding the ICRC study pertains to the nature of some national sources it relies upon. A case in point is the reference to an Israeli “Manual on the Rule of Warfare” – a document that does not actually exist. The said source was an insignificant and non-binding training booklet, out of use for a long time now, which was never intended to express Israel’s practice nor legal positions – let alone constitute a “manual.” Although this mistake was repeatedly notified to the ICRC, it is yet to be corrected, resulting in the Study – as well as scholars relying on the study – basing itself upon an incorrect resource to portray Israel’s legal position. During the Conference, a representative of the Belgian Armed Forces shared a similar story concerning a similarly mischaracterized “Belgian Manual.”

Despite our criticism, the Study represents an impressive feat and it is safe to say a considerable number of the rules in the Study reflect widely-accepted CIL. However, other rules in the study are subject to substantial debate and dispute. Accordingly, a rule-by-rule analysis of the ICRC Study is imperative.

In a recent article, Marko Milanovic and Sandesh Sivakumaran explored the authoritative nature of the ICRC Study, concluding that the study seems to be treated by many actors as highly authoritative. Marko graciously presented his findings during the discussed Conference panel, triggering an interesting debate between the panelists.

We don’t intend to dispute the empirical findings of Marko and Sandesh’s impressive survey. The Study is an exceptionally popular source in LOAC analyses, and the common tendency to cite it as a primary or sole authority for the existence of CIL is evident. We do it ourselves occasionally, and so would other colleagues in the Department. But – and this is a vital point – this would be done only with clearly non-controversial determinations, where the Study offers a convenient footnote for something we have no doubt about (and, obviously, where we simply wish to describe the ICRC’s position). In all other cases, it is necessary to examine the evidence brought to substantiate the purported CIL rule, to determine its accuracy in identifying customary law.

Accordingly, we think that the authoritativeness of the Study should be assessed on a case-by-case basis. The ultimate test of authoritativeness would be in how non-trivial propositions are treated, particularly by users of the study whose position requires them to scrutinize more critically its methodology and conclusions – such as states not bound by a similar provision due to a treaty obligation.

Application of Existing Customary Law to New Domains of Warfare

Our second observation concerns the application of CIL to emerging domains of warfare, where, at least until recently, there has been relatively little to no military activity. This observation is made against the backdrop of recent academic projects on LOAC applicable to cyber operations and outer space activities, and in light of practical experience we’ve accumulated in tackling this question in recent years as part of our work.

CIL is created in context, with state practice typically developing in a particular domain and with relevant opinio juris. It follows that rules of customary LOAC cannot be presumed to automatically apply to new domains. Instead, a case-by-case analysis is necessary to determine whether a specific rule known to be applicable to well-trodden contexts is applicable also in new domains.

Our premise for these analyses is, as always, the two-element approach: First, we examine whether the practice establishing the customary rule is closely related to the scenario in the new domain, as set out in the Lotus (page 21) and the North Sea Continental Shelf cases (paragraph 79). Second, we evaluate whether or not the opinio juris behind the rule is domain-specific. The opinio juris can be considered applicable across different domains if it was intended to be so by the states when they engaged in the relevant practice. This examination of opinio juris may be complex and requires a close analysis of the state intention regarding the rule’s applicability. Generally speaking, it seems that states’ sense of legal obligation with regard to domains that do not yet exist is usually limited. If a customary rule existing in one domain is inapplicable to another domain, we’ll still monitor whether general State practice accepted as law emerges in the latter domain in the future.

An illustrative example of a set of rules that are generally domain-dependent is the law of neutrality, which – as noted by Israel’s Deputy Attorney General in a 2020 speech on legal issues of cyber operations – is specifically tailored to the traditional domains of land, sea and air. The inviolability of a neutral state’s territory is expressed differently in each of these domains. Given these differences, this rule cannot be applied as-is in the cyber domain, and it remains to be seen if and how it would be applied through state practice and opinio juris. An opposite example is the rule of proportionality in attacks, which states have – so far – seemed to apply uniformly in different domains of warfare, thereby giving rise to the general nature of its applicability.

Despite the nuance required in assessing multi-domain applicability, academic works occasionally show a rather lenient approach towards applying existing CIL to an emerging domain, only focusing on the “practical implementation” of the rule in that domain as a matter that requires clarification. Sometimes, such analysis is accompanied by a purported “interpretation” of the CIL rule. This is a separate problem altogether, which I’ll address now.

The (non-)Interpretability of Customary International Law

This post opened with the axiomatic two-element approach to identifying customary international law. The approach essentially provides that states are the critical actors that ultimately determine – through their actions and beliefs – the existence and content of customary law.

Yet, one academic trend that takes direct aim at the status of states as international lawmakers is the notion of “interpreting” CIL. The essence of its proponents’ argument is that identification of customary law in light of state practice and opinio juris should only be a starting point; after the rule comes into existence, its content remains subject to “interpretation.” Although the term “CIL interpretation” has only been coined and developed as a distinct thesis by certain academics in the last few years, it is not a new invention. In the LOAC field, assertions regarding the scope of customary rules that rely on deductive reasoning – rather than on state practice and opinio juris – can be found in various sources, such as academic manuals and other academic publications (see an illustrative example here on page 66). They can be seen sporadically in the ICRC Customary IHL Study, and are increasingly featured in later ICRC publications. For instance, the recently published ICRC Guidelines on the Protection of the Natural Environment follow a relatively consistent formula: They present generally articulated ‘rules’ usually said to have customary status, but proceed to describe their content by interpretatively analyzing a wide variety of sources beyond state practice and opinio juris, including by heavily relying on academic publications and treaty analysis, for example.

The inclination towards “interpretation” may sound appealing at first. Interpretation of rules offers a natural association for us lawyers. There might be a special appeal in the LOAC context due to the close relation between many CIL rules and parallel treaty provisions. Interpretation also offers an undeniable convenience factor – it enables taking a non-controversial vague articulation of a certain customary rule, and then replaces any arduous work of discerning state practice and opinio juris intended to clarify the rule’s content, with deductive reasoning. This approach appeals to lawyers’ intellectual curiosities (and offers certain interpretive freedoms), and avoids conducting historic explorations of conflicts of which lawyers do not necessarily have expertise in.

However, the notion of CIL interpretation is inconsistent with the two-element approach to identifying CIL, as explained by Sir Michael Wood and Omri Sender. It undermines the underlying rationale of custom, which primarily draws its power and legitimacy from what states actually do. The ILC’s draft conclusion 2 of its Draft Conclusions on the Identification of Customary International Law stipulates that the use of state practice and opinio juris serves not only to establish the existence of customary law, but also to define its content. When a phase of interpretation is added to the process, it introduces a new element to the creation of the customary rule.

This concept can also give rise to anomalies and difficulties. Unlike treaties, there is no substantive guidance for interpreting customary law and no agreed text to interpret. It leaves room for far-reaching conclusions, which can deviate significantly from state practice, and subsequently lead to impractical legal consequences.

It is evident that a certain form of interpretation is required in the process of identifying customary law. For instance, one may need to interpret statements to determine the existence of opinio juris, or to interpret the meaning of specific state conduct as relevant practice. However, “CIL interpretation” takes it much further and leads us astray.

Compliance with the existing rules on CIL identification – and preservation of the integrity and legitimacy of CIL as a legal source – requires in our view that that researchers and practitioners exercise caution and avoid taking the “easy” path of relying on interpretation while bypassing the challenges associated with researching state practice and opinio juris.


State legal advisors are regularly required to identify and apply CIL, including customary LOAC. This necessitates adhering to the appropriate methodology of CIL identification and conducting an independent analysis of the existence of customary rules, while also consulting (critically, when needed) works like the ICRC Customary Study. While certain areas of international law, such as jus ad bellum, have benefited from impressive collections and analyses by scholars of state practice, LOAC scholarship lags in this regard. By engaging in the rigorous collection of state practice and opinio juris, we can ensure proper implementation and respect for both LOAC and the accepted rules of CIL identification.

The views expressed in this post are those of the authors and do not necessarily represent those of the IDF or the State of Israel.

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Brian Cox says

August 29, 2023

Excellent insight from a practitioner perspective on a timely topic of increasing importance. If a methodologically rigorous approach to the two-element method of identifying CIL were consistently adopted, it is almost certain that the universe of *truly* customary LOAC rules would be considerably smaller than it appears in contemporary academic literature (including the universe presented in the ICRC CIHL Study).

A useful point of origin for explaining the apparent expansion of customary LOAC rules by interpretation is the ICRC articulation of the purpose of IHL as "a set of rules that seeks, for humanitarian reasons, to limit the effects of armed conflict." This understanding of the purpose of LOAC encourages a progressively restrictive interpretation of applicable rules -- often (I believe) at the expense of methodological rigor when applying the two-element method of identifying CIL.

Too often, discussions among military practitioners don't engage with discussions among humanitarian scholars or civilian protection advocates -- and vice versa. This dynamic contributes to a general lack of understanding of competing perspectives among commentators/practitioners in the two broad communities.

Complements to Col Katzir & Maj David, the EJIL: Talk! editorial team, and the organizers of the IDF MAG LOAC conference for contributing to/creating fora for LOAC commentators and practitioners from different communities to engage with one another and encourage broader understanding of divergent approaches and perspectives. I look forward to seeing additional material based on the IDF MAG LOAC conference if there is more to come from the symposium, and I have no doubt that this sentiment is shared by many other LOAC practitioners and scholars.