Choice of Law in the Laws of War
In private international law, choice of law is a critical question concerned with identifying which legal system governs a dispute involving cross-border elements. A set of principles developed to address such questions does not resolve the dispute itself but rather guides judges toward the applicable substantive law that will determine the outcome.
In public international law, classic choice-of-law questions in this technical sense are rare, although normative conflicts and various methods for their reconciliation have been extensively studied. Nonetheless, a choice of law perspective assumes particular significance in the context of armed conflict, where the issue is not the selection between different legal systems, but the identification and selection of a body of applicable rules—such as the law governing the use of force in international relations, the law of targeting, and the law governing seizure and destruction of enemy property—that regulates distinct aspects of military actions.
This choice-of-law inquiry must be distinguished from questions of interpretation. The former concerns the selection of the relevant legal regime, whereas the latter addresses the meaning and content of rules once their applicability has been established. Confusion may arise, however, where a rule contains a clause that appears to allocate or discriminate between legal frameworks. In such a case, the ultimate choice-of-law outcome may depend on how the clause is interpreted.
This post illustrates the practical significance of this perspective by demonstrating how the choice of law dictates the legal assessment of military action. In particular, it addresses the interaction between the jus ad bellum and jus in bello regimes, the choice between conduct of hostilities rules across different operational domains, and the boundary between the law of targeting and the law governing the seizure and destruction of enemy property.
Jus ad Bellum vs. Jus in Bello
It is a settled proposition of modern international law that military action is governed by two distinct legal regimes: the jus ad bellum, which governs the recourse to force in international relations; and the jus in bello, which regulates the conduct of hostilities once an armed conflict has erupted. Although a single course of military action may constitute a “double trigger” event simultaneously engaging both regimes, the distinction between them is preserved because they address different normative questions (U.S. Department of Defense (DoD), Law of War Manual, § 3.5). The jus ad bellum concerns the legality of a State’s political decision to resort to force at the level of international relations, whereas the jus in bello regulates the lawfulness of individual conduct committed in the prosecution of that decision. Conflating the two regimes not only undermines the structural integrity of the modern laws of war but also risks dire operational consequences by obscuring the distinct legal standards that guide both military decision-making and legal advice for it.
Although oriented toward different goals, the two regimes are not inherently incompatible. There are even instances where legal justifications under the jus ad bellum may inform legal considerations within the jus in bello. Nevertheless, the choice between the two regimes assumes decisive importance where a State seeks to rely on jus in bello justifications for military action that would otherwise contravene the requirements of the jus ad bellum. In such cases, the prior question of which legal regime governs the assessment becomes determinative of legality.
For example, a belligerent party may seek to justify the use of force against an adversary operating within, or from, neutral territory as an exercise of the belligerent right of self-help, arguing that the neutral State is unable or unwilling to prevent violations of neutrality (DoD, Law of War Manual, § 15.4.2). From the perspective of the neutral State, however, such conduct may instead be characterized as a use of force prohibited under international law by electing to apply the jus ad bellum framework. According to this view, the aggrieved belligerent may resort to forcible action only within the confines of the right of self-defense, where the breach of neutral obligations is of such gravity as to constitute an armed attack against it (p. 611).
Similarly, a neutral State’s forcible action against, for example, a drone launched by one of the belligerents might be condemned as an unlawful use of force on the ground that such an aerial incursion does not rise to the level of an armed attack. Yet the neutral State may instead seek to justify its conduct as an enforcement of neutral rights, or as an implementation of its obligations under Articles 5 and 10 of the 1907 Hague Convention V if applicable, which thus cannot be regarded as a hostile act in the jus ad bellum sense. It is also plausible to characterize such action as a form of law enforcement undertaken within the neutral State’s own territory, which does not amount to a prohibited use of force in international relations (p. 101–102).
The significance of this choice-of-law question has featured prominently in recent debates over the legality of U.S. military action against Iran. Numerous scholars and legal practitioners have condemned Operation Epic Fury as a clear violation of the prohibition on the use of force under Article 2(4) of the UN Charter, proceeding on the assumption that the jus ad bellum provides the controlling legal framework. This critique, however, has overlooked, or at least has not fully engaged with, the possibility of evaluating the operation within an alternative legal paradigm by characterizing the whole situation (not just specific strikes conducted during this operation) as the continuation of an existing international armed conflict. As Geoffrey Corn has long pointed out, such a characterization would shift the analytical focus toward the jus in bello regime. Indeed, the legal justification offered by the U.S. State Department’s Legal Advisor, Reed Rubinstein, are grounded in a pattern of sustained Iranian hostilities over decades directed against the United States, Israel, and other countries in the region, noting that defensive U.S. actions could have been “considered part of an ongoing international armed conflict between Iran and the United States itself.”
The legal implication of the latter’s position is that the jus in bello regime governs the whole of this military campaign, relegating jus ad bellum considerations to a secondary role, possibly confined to evaluating the overall proportionality of the campaign as a whole against the threat. The merits of such an approach are, of course, open to debate. However, this disagreement does not negate the need to address the choice-of-law question before proceeding to a substantive legal assessment. Failing to engage with the alternative legal framework risks conclusions that rest on an incomplete or distorted legal foundation.
The Laws of War Across Different Operational Domains
The rules governing the conduct of hostilities vary according to the operational domain in which military activities are carried out. Contemporary rules for the protection of civilians have developed largely in response to aerial bombardment in the context of land warfare, culminating in Section I, Part IV, of Additional Protocol I to the Geneva Conventions. This development, however, did not entirely reshape the pre-existing laws of war, nor did it settle the contours of the legal regime applicable to naval and air operations.
The separation of the legal regimes applicable to each operational domain is expressly recognized in Article 49(3) of Additional Protocol I, which reads:
The provisions of this Section apply to any land, air or sea warfare which may affect the civilian population, individual civilians or civilian objects on land. They further apply to all attacks from the sea or from the air against objectives on land but do not otherwise affect the rules of international law applicable in armed conflict at sea or in the air.
This provision operates as a choice-of-law clause. It delineates the scope of application of the rules contained therein, while deliberately excluding the legal regimes governing conduct of hostilities at sea or in the air from its reach. The drafting history confirms the intention to preserve these domain-specific legal frameworks. Proposals to broaden the scope of this Section by removing the term “on land” were rejected (para. 1893) in order to avoid encroaching upon existing rules applicable to naval and air warfare (p. 330–31). In particular, some delegations considered it undesirable to extend these provisions to activities such as attacks on merchant vessels or civil aircraft, which were regulated by distinct bodies of law such as the rules on blockade or visit and search (para. 21).
Interpreted in light of the drafters’ intent, it would be incorrect to assume the relevant provisions of Additional Protocol I extend to military operations conducted at sea or in the air simply by construing the phrase “may affect the civilian population, individual civilians or civilian objects on land” to encompass economic or financial impact. The rule on effective warning under Article 57(2)(c) employs the same term (“may affect the civilian population”), which indicates that this qualification is triggered only where a naval or air operation may reasonably be expected to cause physical harm to civilians or civilian objects on land.
The Newport Manual on the Law of Naval Warfare shares this understanding (§ 1.2.2), so does the Danish military manual which illustrates the point with examples such as the downing of an enemy aircraft over urban areas or attacks on warships in civilian ports where “harm may be caused to civilians or nearby structures on land” (§ 1.4.1). Although the Harvard Manual on Air and Missile Warfare appears to downplay the significance of this delineation, it is unclear whether the drafters intended to posit the uniform applicability of the entire legal regime across all domains of warfare, or whether their observation was confined to the duty of constant care as a general rule (p. 142).
The significance of this choice-of-law question for the application of Additional Protocol I has been illustrated in recent debates regarding the legality of attacks on undersea infrastructure, such as submarine cables and pipelines. Bill Boothby, for example, approaches this issue as a targeting law question, relying on the broad understanding of the civilian impact on land, and linking it to the ongoing debate over whether war-sustaining objects may qualify as military objectives. James Kraska, by contrast, situates the matter squarely within the framework of the law of naval warfare, while acknowledging that “the content of the law is murky.” Indeed, such operations have historically been treated as the seizure and destruction of property, a distinct body of law which, as discussed below, operates separately from the law of targeting (p. 467–91).
The applicability of this choice-of-law clause to new domains of warfare remains uncertain. Additional Protocol I does not expressly address military operations in the cyber or space domain. However, the same qualification for the choice of land-based rules can plausibly be extended so that its provisions would govern such operations insofar as they may be reasonably expected to cause physical harm to civilians or civilian objects on land. The Tallinn Manual 2.0 appears to have adopted this approach, extending the existing body of conduct-of-hostilities rules to cyber operations when crossing this threshold of physical harm (p. 415). By contrast, the Woomera Manual fails to address this critical question and instead blindly extends the land-based rules to attacks against space objects regardless of physical harm caused in the terrestrial plain (p. 307–308).
Below the threshold of physical harm, distinct, domain-specific (sui generis) rules may conceivably emerge, tailored to the unique characteristics of each operational environment. Recent calls for the development of common understandings regarding the protection of critical infrastructure from cyber operations, such as those advanced by Colombia (p. 11–12) and Pakistan (para. 12), may be seen as indicative of this trajectory of normative development.
Targeting vs. Seizure and Destruction of Enemy Property
Contained in Section I, Part IV, of Additional Protocol I is the body of law governing the conduct of hostilities, commonly referred to as the law of targeting. This body of law emerged from the practice of aerial bombardment and the gradual efforts to regulate it, culminating in the development of a distinct set of rules regulating attacks against the adversary. It is a special legal regime carved out from a more rudimentary legal framework governing battlefield seizure and destruction in general. The latter regime centers around Article 23(g) of the 1899/1907 Hague Regulations, which prohibits the seizure and destruction of enemy property unless imperatively demanded by the necessities of war.
Jeffrey Lovitky has previously provided a detailed account of the distinction between the two legal regimes, which need not be rehearsed here. Instead, three points may be highlighted to show the significance of the choice-of-law question at play. First, as explored in my forthcoming work, the law governing seizure and destruction is rooted in the concept of property rights, and thus its focus is fundamentally different from the law of targeting, which is concerned with the protection of civilians and civilian objects from the effects of hostilities. Second, the law of targeting applies to attacks conducted at a distance against the adversary, whereas the law of seizure and destruction governs the broader category of military operations involving the appropriation of, or damage to, enemy property; for example, burning, demolishing, or otherwise damaging property to clear a field of fire, to provide material for military works, or to facilitate withdrawal. Third, military necessity is generally sufficient to justify seizure and destruction (with the qualification “imperatively demanded” imposing no additional constraint in practice), which means that even civilian objects lacking direct military use may be subject to lawful seizure or destruction on grounds of military necessity.
Despite these clear differences in the scope and function of each legal regime, the resulting choice-of-law question has been poorly addressed in the recent practice of international courts and tribunals. In the Orić trial judgment, for example, the Yugoslav Tribunal condemned Bosnian Muslims for the deliberate burning of civilian property after hostilities had ceased, finding that “at the time of the attack, the property destroyed … was neither of a military nature, nor was it used in a manner such as to make an effective contribution to the military actions of the Bosnian Serbs” (para. 607). Similarly, the Naletilić trial judgment dismissed claims of military necessity by observing that the destruction of property was committed after the cessation of shelling (para. 589). By failing to engage with the choice-of-law question, the tribunal effectively evaluated acts of seizure and destruction within the framework of targeting law, thereby conflating two distinct legal regimes.
An opportunity to clarify this confusion arose before the International Criminal Court when it dealt with the choice-of-law question in the trial of Bosco Ntaganda. Drawing on amicus submissions, the Appeals Chamber undertook a detailed examination of the definition of “attack” as the threshold inquiry for determining the applicable legal regime. With the majority adopting a narrow construction of attack limited to combat action (paras. 1141–42), the rebel commander was ultimately acquitted of the charge of attacking protected objects. As Judge Balungi Bossa observed, however, the outcome might well have been different had the charge been framed instead under the war crime of seizure and destruction of enemy property (paras. 12, 15).
Despite the prominence of Ntaganda in illustrating the significance of the choice-of-law question, the Court reverted to conflating the two legal regimes in the Ali Kushayb judgment. This time, the Janjaweed commander was appropriately charged with the war crime of wanton destruction in the context of a non-international armed conflict arising from the destruction of civilian villages. However, as Evin Stovall and I recently argued, the judgment assessed the conduct by inquiring whether the destroyed property constituted a military objective, treating it as a protected object unless proven targetable (paras. 727, 857). Such a misapplication of the legal standard, especially in the wake of Ntaganda’s acquittal, amounts to a particularly egregious doctrinal error.
Concluding Observations
This post has highlighted three distinct choice-of-law questions that are critical to the legal assessment of military action, but these are by no means exhaustive. Other interfaces between legal regimes, whether temporal, functional, or domain-specific, may similarly give rise to competing characteristics and divergent legal outcomes. What these examples demonstrate, however, is that the identification of the applicable legal framework is not merely a preliminary technical exercise, but a determinative step that shapes the entire legal structure of analysis.
In the absence of a shared understanding of how such choice-of-law questions are to be resolved, strategic considerations are likely to drive legal characterization. States may invoke the legal framework most favorable to the justification of their conduct, while critics condemn that same conduct by situating it within an alternative, but equally plausible, regime. The result is not only the potential mischaracterization and wrongful condemnation of particular acts, but also the entrenchment of competing narratives that lack a common analytical foundation.
This dynamic risks reducing legal debate to an exchange of assertions rather than reasoned disagreement within a shared framework. Unless clear guidance merges, whether through coherent jurisprudence of international courts and tribunals or through the convergence of State practice, this problem will continue to undermine the normative authority of international law as an instrument for the regulation of warfare.
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Hitoshi Nasu is a Professor of Law in the Department of Law and Philosophy at the United States Military Academy.
The views expressed are those of the authors, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.
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Photo credit: U.S. Navy, Navy Petty Officer 3rd Class Aaron Haro Gonzalez
