Lieber Studies Making and Shaping LOAC Volume – The Status and Influence of Expert Manuals

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.

Since the publication of the International Institute of Humanitarian Law’s San Remo Manual on Armed Conflicts at Sea in 1995, ad hoc expert groups of academics, non-governmental practitioners, and government legal advisers (usually acting in their personal capacity) have produced an array of unofficial manuals intended to restate existing treaty and customary law. Notable among these are the 2006 Manual on the Law of Non-International Armed Conflict (NIAC Manual); the 2010 Manual on the International Law Applicable to Air and Missile Warfare (AMW Manual); the 2013 Tallinn Manual on cyber warfare and its 2017 update, Tallinn Manual 2.0 on cyber operations; the 2017 Leuven Manual on the International Law Applicable to Peace Operations; the 2020 Oslo Manual on Select Topics of the Law of Armed Conflict; the 2023 Newport Manual on the Law of Naval Warfare; and the 2024 Woomera Manual on the International Law of Military Space Activities and Operations (I participated in the NIAC Manual, AMW Manual, and Woomera Manual projects, and directed the Tallinn Manual efforts).

The manuals generally consist of “rules” that purport to reflect the extant law and commentary that explains them. It is the commentary that is most normatively significant, for it is there that the experts take on uncertainty or disagreement over the content, interpretation, and application of the rules.

Although such efforts are currently in vogue, expert manuals are not new. On the contrary, expert manuals have influenced the development of the law of armed conflict throughout its modern era. For instance, in 1880, the Institut de Droit International published the Oxford Manual of the Laws and Customs of War, which, together with the unratified 1874 Brussels Declaration, heavily influenced the Regulations annexed to the 1899 Hague Convention II and 1907 Hague Convention IV. The Nuremberg Tribunal and International Court of Justice subsequently recognized their customary law status, with the latter characterizing them as “fundamental rules” reflecting “intransgressible principles of international customary law” (Trial of the Major War Criminals, vol. 1, p. 254; Nuclear Weapons Advisory Opinion, para. 79).

The central issues posed by all such efforts are their status in international law and the extent of their influence on that body of law. In other words, what is their normative impact as a matter of both law and fact? This was the question I addressed in my contribution to Making and Shaping the Law of Armed Conflict and to which I return here.

Legal Status

There is no question that the influence of scholars on international law has diminished over time. Many factors explain this decline, including the adoption of treaties, the cacophony of commentary that modern publishing technology makes possible, and the ever-growing size of the international law scholarly community. The latter two factors have made it challenging to separate analytical wheat from chaff, while the first means there is less need for reliance on scholarly works when examining international law.

Nevertheless, as a matter of law, scholarship still matters. Article 38(1)(d) of the Statute of the International Court of Justice provides that the “teachings of the most highly qualified publicists [scholars] of the various nations” are “subsidiary means for the determination of rules of law.” Thus, although they do not create binding obligations, expert manuals by “highly qualified publicists” are, by law, consequential when identifying and understanding international law rules. In this regard, the International Law Association has observed, “The reference to ‘the most highly qualified’ publicists emphasizes that attention ought to be paid to the writings of those who are eminent in the field. In the final analysis, however, it is the quality of the particular writing that matters rather than the reputation of the author” (Identification of Customary International Law, para. 4 of commentary to draft conclusion 14; see also Paquete Habana, p. 700).

Accordingly, international tribunals have long cited scholarly works. For instance, the Permanent Court of Justice did so to confirm exclusive flag State jurisdiction in its 1927 S.S. Lotus judgment (para. 68), while the International Court of Justice looked, inter alia, to editions of Oppenheim’s International Law on the issue of historic bays (Land, Island and Maritime Frontier Dispute, para. 394). So, too, have the European Court of Human Rights and the International Criminal Tribunal for the former Yugoslavia. Domestic courts likewise sometimes turn to scholarship when taking on international law issues, as in the case of Israel’s Supreme Court considering the NIAC Manual in its Public Committee against Torture (Targeting Killings) judgment (para. 21).

This raises the question of whether greater Article 38(1)(d) weight should be accorded to expert manuals than other forms of scholarship. In this regard, Sir Michael Wood has opined that “it is surely the case that more authority attaches to the coincident views of writers than to controversial or speculative positions” (Max Planck Encyclopedias of Public International Law, Art. 38(1)). I agree for four reasons. First, while expertise matters more than numbers, the number of experts adopting a view (as in a group) logically counts. Second, meetings that lead to the promulgation of expert manuals tend to involve a great deal of give and take, during which the group teases out the strengths and weaknesses of positions. Third, in almost every case, the drafting of an expert manual is measured in years, thereby allowing for considerable reflection before publication. Finally, expert groups tend to involve participants from a variety of nations. They bring different perspectives to issues, enriching the “marketplace of ideas” dynamic of such groups.

It appears some States, at least to an extent, agree. Concerning the law of armed conflict, for instance, the UK Manual on the Law of Armed Conflict and Canada’s Law of Armed Conflict Manual rely heavily on the San Remo Manual, and the Danish Military Manual repeatedly cites Tallinn Manual 2.0, the AMW Manual, the NIAC Manual, and the San Remo Manual.

Because most of the expert manuals cited above qualify as the teachings of highly qualified publicists, using them when identifying, interpreting, and applying international law rules is technically appropriate. But the question remains whether, and if so why, they actually exert a meaningful influence on State understandings of international law.

Influence

In my opinion, “influence” denotes an impact on State understandings of international law, for only States authoritatively make and interpret international law. Thus, a manual’s influence on international law depends primarily on how States receive it.

Such influence can manifest in at least three ways. First, a manual can influence States to acknowledge the existence of a particular rule. The paradigmatic example is the rapidly growing acceptance by States of the two Tallinn Manuals’ assertion that a rule of sovereignty governs State actions in cyberspace (Tallinn Manual 2.0, rules 1-5). Although this may have been debatable in the past, the contrary view is an outlier among States today.

Second, expert manuals can influence a State’s understanding of international law rules. This is the most prevalent form of expert manual influence. Consider the U.S. position on “war-sustaining” military objectives (Department of Defense, Law of War Manual, § 5.6.6.2). Most manuals dealing with the law of armed conflict have rejected this position, thereby bolstering the willingness of States to do likewise. Resultantly, the war-sustaining approach is, like the rejection of sovereignty, a relative outlier.

Third, expert-driven processes are especially useful in identifying interpretive issues. For example, the Tallinn Manual 1.0 experts were the first to draw significant attention to the questions of how the law of armed conflict term “attack” should be understood in the cyber context and whether data qualifies as an “object” with respect to the prohibition on attacking civilian objects (see discussion here).

Lastly, manuals can suggest innovative ways to interpret international law rules that enable them to maintain continued relevance in the changed context in which they are being applied. For example, the Tallinn Manuals proposed applying a consequence-based approach in use of force determinations involving cyber operations, adapting the self-defense “scale and effects” test to cyber uses of force assessments (see Paramilitary Activities, para. 195) and extending the notion of damage in the law of armed conflict to cyber infrastructure loss of functionality. All three suggestions are finding increasing favor with States.

Of all the expert manuals, the Tallinn Manuals have proven the most influential. Not only have they shaped much of the debate over how international law governs cyberspace, but Tallinn Manual 2.0 has become a (“the” for many States) “go-to” reference point for States when dealing with the topic. This is the case with respect to the promulgation of official positions on: the applicability of international law in the cyber context; diplomatic negotiations among States, such as those that have taken place in the multiple UN Groups of Governmental Experts and the ongoing Open-Ended Working Group; guidance to government ministries and the armed forces; and legal advice during cyber-related events. To highlight but a few examples, France’s Manual on the Law of Military Operations and Denmark’s Military Manual cite Tallinn Manual 2.0. The French Ministry of the Armies addressed the Manual in its comprehensive position on international law. The Netherlands Ministry of Foreign Affairs referred to Tallinn Manual 2.0 in multiple legal policy statements. Australia cited it in its “International Cyber and Critical Technology Engagement Strategy.” Meanwhile, the European Parliament passed a resolution acknowledging the relevance of the publication. And numerous States have based positions on the Manual during UN activities. Anecdotally, the influence of Tallinn Manual 2.0 behind-the-curtain appears to have been even greater.

This begs the question of why the Tallinn Manuals have proven so influential. There are a number of reasons, some of which have contributed measurably to the success of other manuals. First, and perhaps most importantly, timing can be (almost) everything. When the experts began their work in 2009, there was almost no serious scholarship on, or State consideration of, how international law applied in the cyber context. And following cyber shocks to the system in Estonia (2007) and Georgia (2008), States were urgently trying to figure out how to characterize cyber operations under international law. The first Tallinn Manual provided comprehensive guidance on most issues raised during those events, and Tallinn Manual 2.0 built that guidance out across peacetime law. Quite simply, in the land of the blind, the one-eyed man is king.

Second, both manuals adopted an approach to the commentary by which all reasonable views on unsettled issues would be set forth objectively (although majority and minority views were identified). As a result, States seldom felt boxed in by the manuals. On the contrary, they could usually find legal support for their position, even if it appeared only in a minority, but by definition reasonable, view.

Third, the project leadership worked hard to secure State buy-in throughout the drafting process. In particular, the Netherlands Ministry of Foreign Affairs generously convened multiple meetings (the “Hague Process”) where the leadership met with representatives from over 50 States and international organizations to secure their thoughts on draft rules and commentary. In many cases, they adjusted the text to accommodate State input, lending credibility to the process and product.

Fourth, the project made a concerted effort to create a true marketplace of ideas. In this regard, the project included experts from diverse geographical and legal backgrounds. Moreover, scores of legal and technical experts from around the world provided extensive peer review. This effort not only dramatically improved the quality of the text but also enhanced its standing. So, too, did the fact that some group members were current or former State legal advisers. Although participating in their personal capacity, they ensured the final product was firmly grounded in the reality of State legal practice.

Finally, at the initiative of the Netherlands, the project crafted a robust capacity-building program around Tallinn Manual 2.0 (the idea came from the Swiss government’s sponsorship of training based on the AMW Manual). Conducted initially by expert group members, week-long courses in international cyber law for government officials took place worldwide. Notably, the experts did not “pitch” particular manual findings. Instead, they laid out the various interpretive options that States could consider for adoption consistent with their individual national interests, as had been done in the commentary. This approach further boosted the work’s credibility. Today, Canada, Singapore, and Australia have joined the Netherlands in building cyber law capacity in other States.

Positive or Negative Influence

Perhaps most importantly, it must be cautioned that the mere fact of influence is not necessarily positive or negative. On the positive side, expert groups enjoy significant leeway to examine the law objectively. States, by contrast, are limited by national interest and policy constraints. Moreover, the participants are, after all, “experts” in the subject matter. With a few exceptions, States are unlikely to field comparable expertise (with respect to both quantity and quality) in their legal divisions. After all, most legal advisors shoulder multiple portfolios, thereby limiting their ability to master a narrow subject area. The tyranny of workload further hinders them.

However, expert-driven processes are not a panacea, for they can negatively influence the law. The participants may be experts, but this does not mean they are bias-free. The paradigmatic example is the existence of competing law of military space operations expert groups. MILAMOS and Woomera Manual project experts had originally comprised a single group but split when some members (including myself) felt that others were insufficiently objective. Properly composed, such groups should consist of individuals who approach the topic from different perspectives but are willing to “receive” and not just “transmit” during discussions. Few expert groups are entirely successful in this regard.

Many other obstacles to success exist. Some groups are subject to the dominant influence of one or more individual experts. Others fall prey to “groupthink.” And, if we are being honest, groups are sometimes built with factors other than merit in mind, which can undercut the synergistic effect of group deliberations. The result is that there are some true expert groups and others that merely label themselves as such; most fall somewhere between these extremes.

Concluding Thoughts

The reader can be excused for assuming I am a fan of expert manuals. But I am not, at least not in the abstract. As I emphasized in my chapter of the book,

Only States may authoritatively interpret international law, and it is only when their interpretations achieve a critical mass that legal clarity is achieved. Of course, the teachings of publicists have a formal role in the process, as noted in Article 38(1)(d) of the Statute of the International Court of Justice. But at the end of the day, State positions matter more than scholarly opinions. This is especially so in light of war’s complexity, which is sometimes difficult to grasp without operational understanding.

So, I wish States would take the lead in addressing novel or challenging international law issues. Yet, for the reasons outlined above and more, States often fail to engage in the in-depth legal analysis necessary to adopt definitive legal positions. It is only natural that scholars will attempt to fill that void. In my opinion, this usually is best done by groups of experts operating in a give-and-take dynamic that allows normative cream to rise to the top.

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Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy at West Point. He is also Professor of Public International Law at the University of Reading and Professor Emeritus and Charles H. Stockton Distinguished Scholar-in-Residence at the United States Naval War College.

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