Editor’s Note: The U.S. Department of Defense’s (DoD) Law of War Manual, first published in 2015, sets out the DoD’s authoritative interpretation of international law on matters relating to armed conflict. In recognition of its size, breadth, and the evolving nature of law and conflict, the Manual is also a living document that anticipates needing revisions and updates. In A Living Document, published by Just Security and the Reiss Center on Law and Security, leading scholars and practitioners reflect on how the Manual is keeping pace with its stated purpose and examine specific topics and rules where the Manual may be ripe for clarification or strengthening.

The Department of Defense Law of War Manual (June 2015; updated July 2023)  (Manual) impressively catalogues, over more than 1,200 pages, the U.S. Department of Defense’s position on the law of war, including on the conduct of hostilities, weapons, the treatment of prisoners of war, military occupation, neutrality, and cyber operations. In explaining the rules and policies that are to guide DoD personnel, the Manual relies heavily on treaties and customary international law, which prompts consideration as to the methodology used when applying those sources, and whether improvements to the Manual are warranted in that regard.

As discussed further below, such improvements could include, for example, explaining the interrelationship of the law of war with fields such as international environmental law or the law of the sea; identifying the practical implications in this area of the concepts of jus cogens and “specially affected” States; and addressing whether a major multilateral treaty can lead, over time, to the emergence of customary rules binding upon even non-parties to the treaty and, if so, whether that has happened for the United States with respect to the most significant rules of the law of war.

Methodology for Determining U.S. Treaty Obligations

Several components feature in the methodology used by the Manual when applying treaties. First, the Manual properly recognizes the relevance of the United States being a party to the treaty and, if a non-party, whether the United States is a signatory. Indeed, a documentary appendix to the Manual (pp. 1161 et seq.) identifies the relevant treaties to which the United States is a party, such as the 1907 Hague and 1949 Geneva Conventions and the Biological Weapons Convention. Further, the appendix notes examples of treaties signed but not ratified by the United States, such as Additional Protocols I and II to the 1949 Geneva Conventions (API and APII), the Vienna Convention on the Law of Treaties (VCLT), and the Rome Statute of the International Criminal Court, as well as examples of treaties that the United States has neither signed nor ratified, such as the Ottawa Landmines Convention. (At § 19.2.3, it erroneously lists the United Nations Convention on the Law of the Sea as “signed but not ratified”; the United States has done neither.) Certain treaties and documents of historical value are also listed, such as the Lieber Code adopted during the U.S. civil war. All these instruments feature, to a greater or lesser degree, throughout the Manual.

Second, the Manual considers the relationship of treaties inter se. For example, the Manual properly identifies that later-in-time treaties of the United States can overtake earlier treaties, in relations between the parties, such as the 1949 Geneva Conventions vis-a-vis their forerunners (pp. 1185-87). Further, the Manual considers the relationship between law of war treaties (and other rules) and treaties arising in other areas of international law, notably international human rights law (IHRL) (§ 1.6.3), with an emphasis on law of war rules being the lex specialis (§ and on interpretive constructions that avoid conflicts (§ Greater attention in this regard might be paid to other fields of international law, such as international environmental law, which may both inform law of war instruments (such as the meaning of the term “environment”) and conflict with them (for an effort to harmonize, see the International Law Commission (ILC)’s 2022 Principles on Protection of the Environment in Relation to Armed Conflicts, with commentary, and the ICRC’s 2020 Guidelines on the Protection of the Natural Environment in Armed Conflict). Further, greater attention might be paid to the potential application of other fields when armed hostilities are not directly at issue – such as during a lengthy military occupation, during a “peace operation” (i.e., deployments intended to monitor a peace but that may entail a use of force), or during post-conflict remediation on land or at sea (e.g., clearing mines or remnants of war) – where the application of the law of war as lex specialis seems less obvious. Such attention may be warranted, at a minimum, so as to educate DoD personnel as to the views that coalition partners might take in certain situations.

Third, the Manual properly identifies situations where the United States, although a party to a treaty, has taken a reservation (such as to CCW Protocol III on Incendiary Weapons, see §, has issued an understanding (such as to CCW Protocol IV on Blinding Laser Weapons, see § 19.21.5), or has issued some other kind of statement (such as a certification to Congress that the Chemical Weapons Convention does not prevent the use of riot control agents in certain cases, see § 6.16.2).

Fourth, the Manual recognizes that, under VCLT Article 18, a signatory (even though a non-party) “is obliged to refrain from acts which would defeat the object and purpose of a treaty” (p. 1165, fn. 35). Yet that recognition does not actually feature in the Manual’s analysis of the U.S. relationship to specific provisions of relevant treaties, such as API and APII, both of which the United States has signed. A State can avoid the effects of VCLT Article 18 by making “its intention clear not to become a party to the treaty,” but the U.S. government apparently has not informed the depositary for API or APII (the Swiss Federal Council) that it no longer intends to ratify such conventions, and no notation to that effect appears on the ICRC’s website. Perhaps the U.S. intention not to ratify API is regarded as clear from the Reagan Administration’s well-known decision not to submit it to the Senate (“Protocol I is fundamentally and irreconcilably flawed”), but APII was submitted, and statements as recent as 2020 indicate a continuing Executive Branch desire to become an APII party. In short, the Manual recognizes the relevance of VCLT Article 18, but does little to clarify for DoD personnel its application in practice.

Fifth, the Manual identifies a few other relevant aspects of the law of treaties, such as the possibility of U.S. withdrawal (§ 1.7.3), but other aspects are neglected that might be helpful to DoD personnel, including rules on observance and application of treaties (VCLT Arts. 26-30). The most noticeable is the lack of attention to the methodology for interpreting treaties (VCLT Arts. 31-33). One element of the interpretive process is highlighted in the Manual – the use of subsequent practice in treaty interpretation (§ 1.7.4) – but even there the analysis could be improved by noting the importance of determining whether that practice is of all the parties to the treaty, with practice comprising both action and (in some circumstances) inaction. The ILC’s 2018 Conclusions on Subsequent Agreements and Subsequent Practice in Relation to the Interpretation of Treaties is a useful recent assessment of this particular interpretive element. A future revision of the Manual might also take into account the 2018 Restatement (Fourth) on the Foreign Relations Law of the United States, which addresses in depth the U.S. position on treaty law.

Sixth, where the United States is not a party to a treaty, the Manual occasionally identifies situations where provisions of the treaty remain relevant (see § 3.1), notably: (a) where the provision is also incorporated in other treaties to which the United States is a party (e.g., API Art. 52, §§, 5.6.3); (b) where the provision reflects customary international law (e.g., the VCLT, § 19.18, which is then relied upon about 20 times in the Manual); (c) where the provision is consistent with long-standing U.S. practice (e.g., API Art. 36, § 6.2.3); (d) where the United States supports the provision as a matter of national policy, always or at least in some contexts (e.g., API Art. 75, §; or (e) where the provision represents “modern international public opinion” (§  A future revision might more systematically consider whether any such situation applies for the most salient provisions of law of war treaties, especially API and APII.

Conversely, the Manual indicates some provisions of treaties to which the United States is not a party and to which it objects, such as provisions within API on reprisal, on certain environmental protection, or on works and installations containing dangerous forces. Alas, the Manual does not provide a comprehensive list of the treaty provisions to which the United States objects. A comprehensive list might have benefits (especially with respect to API and APII) in helping to maintain a persistent objector status (see below) and for informing DoD personnel as to the existence of the objections. At the same time, a comprehensive list might be difficult to complete and may even inhibit DoD’s position from evolving over time. If so, the next version of the Manual might at least contain a non-exhaustive list of the most significant treaty provisions to which the United States objects, perhaps with text explaining the possibility for their withdrawal in the future.

Finally, even where a treaty, other rule, or policy does not apply, the Manual does not approach the matter as presenting a vacuum (a non liquet) in which anything goes. Rather, it takes the well-advised approach that, even if an express prohibition is lacking, “the principles of the law of war form the general guide for conduct during war” (e.g., §§, 1.7.3, 1.8.1,, at times referencing the Martens clause (§ In this regard, the Manual could be clearer as to whether it regards such “principles” as emanating solely from treaty or customary international law (implied by the opening text of § 1.3) or as arising from the third main source of international law, general principles of law recognized by States (indicated by § 2.1.1.), given that the latter requires a very different methodology for its identification (see U.N. Doc. A/CN.4/L.982 (2023)).

Methodology for Determining U.S. Customary International Law Obligations

The Manual explains the two traditional components for identifying a rule of customary international law: a general and consistent practice of States in conjunction with opinio juris (§ 1.8). In the context of assessing State practice, the Manual properly stresses the importance for the law of war of assessing actual operational practice (as opposed to simply manuals or official statements) (§, but the Manual should recognize other forms that State practice can take, such as “diplomatic acts and correspondence; conduct in connection with resolutions adopted by an international organization or at an intergovernmental conference; conduct in connection with treaties; executive conduct . . . ; legislative and administrative acts; and decisions of national courts (see ILC’s 2018 Conclusions on Identification of Customary International Law, Conclusion 6(2)). Moreover, it could note that there is no hierarchy as among these forms of practice (id., Conclusion 6(3)).

Other traditional concepts concerning customary international law are presented, but might be improved. The concept of the persistent objector – incapable of preventing the formation of a rule of customary international law, but capable of preventing that rule from applying to the objecting State – is well captured (§§, 1.8.4). Yet some attention should be paid to the concept of jus cogens (peremptory norms of general international law), which is commonly viewed as including “the basic rules of international humanitarian law” and as precluding persistent objection (see ILC’s 2022 Conclusions on Jus Cogens, Annex, para. (d)). The Manual could, for example, acknowledge the concept of jus cogens and its application to “basic rules,” but then indicate what DoD regards those rules to be. Separately, the concept of “specially affected States” has a certain lineage in case law and doctrine (§, but today many States view it as a difficult concept to apply in context, even with respect to the law of war (given that all States, to a greater or lesser degree, tend to be affected by warfare). For some States, that concept is simply a gambit for the major powers to claim a veto or other forms of influence over the emergence of a rule of customary international law. Interestingly, the Manual notes but does not appear to employ the concept in any specific context.

The Manual’s consideration of the important relationship between treaties and customary international law might also be seen as lacking, and here two aspects bear noting. First, the Manual approaches the issue statically, regarding a treaty provision as: reflecting custom; not reflecting custom; or reflecting custom but not precisely (§ 1.8.1.). The principal lacuna here is the failure to consider the emergence of custom over time. As classically stated by the ICJ in the North Sea Continental Shelf cases (paras. 60-62, 71), a treaty at the outset might codify customary international law or might crystalize a customary international law rule that had not yet fully emerged, but a major multilateral treaty might also lead to the emergence over time of rules of customary international law. Indeed, this possibility is recognized in the Judge Advocate General (JAG)’s Operational Law Handbook (2022) when it says: “IHRL treaties can codify existing customary IHRL. Conversely, practices established in treaties can ripen into customary IHRL.” Yet in the Manual, this possibility is unaddressed. Thus, it is not really sufficient, when considering which armed groups are part of a State’s armed forces, to say that API did not codify customary international law as of 1977 (§; whether or not that is true, the question is whether today – almost 50 years later, in light of 174 ratifications/accessions, and the application of API in many respects even by non-parties – API Articles 1(4) and 43-44 reflect contemporary customary international law. It may not, or if it does the United States may be a persistent objector to such provisions, but the dynamic temporal aspect of custom formation should be considered.

Second, the Manual maintains that certain IHRL treaties do not apply extraterritorially (§ or apply only in territory under U.S. jurisdiction (§, but does not make a point of informing DoD personnel that some U.S. military manuals have taken the position that customary IHRL does apply extraterritorially. For example, the Judge Advocate General (JAG)’s Operational Law Handbook (2015) provides: “In contrast to much of human rights treaty law, fundamental customary IHRL binds a State’s forces during all operations, both inside and outside the State’s territory.” If such a handbook reflects DoD’s position, this should be acknowledged in the Manual, and pertinent IHRL treaty rules should be assessed as to their status as custom. Any concerns regarding overly broad coverage of customary human rights law might be cabined by reference to non-derogable human rights or to a listing such as that found at § 702 of the Restatement (Third) on the Foreign Relations Law of the United States.

Of course, when identifying a rule of customary international law, the evidence relied upon is critical. The Manual admirably explains its methodology: for the types of evidence selected; for incorporating footnotes; and for signals within footnotes, such as the meaning of “see,” “see also,” or “consider” (§ 1.2). Yet the Manual’s actual use of the evidence cited might be more rigorous. For example, at times, the Manual neglects to recognize that the United States has already acknowledged a particular rule as customary in nature (such as Article 3(8) of the Amended Landmines Protocol, addressed at §, or fails to capture what the cited evidence actually says (such as the Manual habitually referring to taking “feasible precautions,” when the cited evidence often says “all feasible precautions”).

In any event, the evidence relied upon in the Manual (other than treaties) is dominantly from within the United States, whether it be U.S. government statements, laws, cases, or secondary sources. This is not to say that foreign evidence is excluded; it is used, but sparingly and thus unevenly. A focus primarily on U.S. sources is understandable if the purpose of the Manual is solely to explain the DoD position on customary law of war (§ 1.1.1). Moreover, the focus is understandable given that a broader approach could lengthen the Manual (perhaps considerably) and increase the difficulty of the exercise, given the enormous range of foreign sources, including those not in English.

Yet, in this era of inter-State coalitions and partnerships, more treatment of foreign sources should be considered, including the positions taken by foreign governments, the ICRC, or international courts and tribunals, so as to guide DoD personnel as to the views of others. Moreover, it must be acknowledged that, by focusing primarily on U.S. sources, the Manual limits its significance. The Manual is clearly an example of DoD practice – pertinent for guiding DoD personnel and pertinent for others who might add this unit of U.S. practice to the practice of the United States as a whole, and then to that of other States, so as to identify a rule of customary international law. Yet the Manual cannot be seen as a subsidiary source of international law; it does not itself demonstrate the existence of rules of customary international law. Perhaps the most unfortunate aspect of this limitation is that, where the Manual expressly takes exception to a particular rule as constituting customary international law, there is no or little explanation as to why that is the case, for a standard customary international law analysis (assessing contemporary State practice and opinio juris on the issue) is not undertaken.

Conclusion on How the Manual’s Methodological Approach Might Be Improved

The above suggests a few ways that the Manual might be improved in its methodological approach to treaties and customary international law. The most important areas for greater guidance might be on: (1) the interrelationship of the law of war with other fields of international law, beyond IHRL, such as international environmental law, law of the sea, international criminal law, and law of immunity – including in situations where active hostilities are absent; (2) the significance in practice, if any, of VCLT Article 18, especially as it relates to API and APII; (3) rules concerning the observance, application, and interpretation of treaties; (4) the practical implications in this area of the concepts of jus cogens and “specially affected” States; (5) whether a major multilateral treaty can lead, over time, to the emergence of customary rules binding upon even non-parties to the treaty and, if so, whether that has happened for the United States with respect to the most significant rules of the law of war and IHRL treaties; and (6) whether certain rules of IHRL, though not applicable extraterritorially as treaty law, nevertheless regulate extraterritorial conduct as customary international law.

More generally, for the most salient provisions of the law of war (including those found in API and APII), consideration might be given to applying systematically a checklist that goes beyond simply asking whether the United States is a party to the treaty, but also asks whether the provision: (a) is incorporated in other treaties to which the United States is a party; (b) reflects customary international law; (c) is consistent with long-standing U.S. practice; (d) is supported by United States as a matter of national policy; or (e) represents “modern international public opinion.”