Self-Defense Without Consent: Selective Sovereignty in the Law of War
Abstract
This article examines the growing practice of invoking self-defense to justify the use of force in another state’s territory without its consent, particularly in operations against non-state armed groups. It argues that contemporary interpretations of Article 51 of the UN Charter, especially through the “unwilling or unable” doctrine, reveal a pattern of selective sovereignty in which territorial integrity is formally affirmed but unevenly applied in practice. By analyzing state practice, ICJ jurisprudence, and the interaction between jus ad bellum and the law of armed conflict, the article assesses the legal and operational implications of this evolving approach to cross-border force.
States increasingly justify the use of force in another state’s territory without consent by invoking the right of self-defense under Article 51 of the UN Charter. From U.S. operations against ISIS in Syria to Turkish cross-border strikes in Iraq and Russian claims regarding Ukraine, sovereignty is treated not as an absolute barrier but as a conditional constraint. This article argues that contemporary practice reveals a pattern of selective sovereignty: territorial integrity remains formally affirmed in international law, yet is practically overridden when powerful states invoke necessity, unwillingness, or incapacity. The result is not the erosion of sovereignty in the abstract, but its uneven application within the jus ad bellum and its interaction with the law of armed conflict.
Sovereignty and the Use of Force: A Legal Baseline
Article 2(4) of the UN Charter establishes territorial sovereignty as a foundational constraint on the use of force, prohibiting states from employing military force against the territorial integrity or political independence of another state. Article 51 permits an exception only in cases of self-defense following an armed attack, subject to the requirements of necessity and proportionality. As the International Court of Justice has repeatedly affirmed, these requirements apply regardless of the identity of the attacker, including non-state actors.
However, contemporary state practice increasingly stretches this framework. In operations against non-state armed groups, states have asserted a right to use force on foreign territory without consent when the territorial state is deemed “unwilling or unable” to address the threat. As Deeks notes, this test was originally articulated as a limiting mechanism tied to necessity, not as a stand-alone legal doctrine authorizing unilateral force. When necessity is asserted without meaningful assessment of the territorial state’s capacity or willingness, sovereignty ceases to function as a uniform legal constraint and instead becomes contingent on power and narrative framing.
Is “Unwilling or Unable” Law?
Despite its frequent invocation in contemporary self-defense claims, the “unwilling or unable” test does not enjoy settled status as a rule of customary international law. No treaty provision expressly authorizes the use of force on another state’s territory absent consent on the basis of unwillingness or inability, and international courts have not endorsed the doctrine as an independent legal standard. Instead, the test has emerged through state practice and legal argumentation, leaving its precise content and legal force contested.
Proponents of the doctrine generally present it as an extension of the necessity requirement inherent in Article 51 of the UN Charter, arguing that self-defense is permissible only where the territorial state cannot or will not suppress the threat posed by a non-state actor. On this view, unwillingness or inability does not displace sovereignty, but conditions when the use of force becomes necessary. Importantly, this framing treats the doctrine as a limiting principle, intended to restrain unilateral action rather than license it.
However, the absence of agreed criteria for assessing unwillingness or inability has generated significant legal uncertainty. As Deeks has noted, state practice reveals wide variation in how necessity is evaluated, with some states providing detailed factual justifications while others rely on conclusory assertions. This lack of consistency complicates efforts to identify a coherent customary rule and raises concerns about self-judging determinations that undermine reciprocal constraint.
The International Court of Justice has further refrained from endorsing an expansive reading of self-defense against non-state actors that would weaken territorial sovereignty. In cases such as Nicaragua and Armed Activities, the Court emphasized that necessity and proportionality remain strict requirements, even when armed attacks originate from non-state groups. While these decisions do not foreclose all extraterritorial self-defense, they caution against interpretations that treat sovereignty as presumptively overridable.
Taken together, existing doctrine supports a restrained understanding of “unwilling or unable” as a context-dependent argument within necessity, not as a settled legal rule. Treating the doctrine as law risks overstating its normative force and obscuring the evidentiary burden required to justify violations of territorial sovereignty. This doctrinal indeterminacy provides the foundation for selective sovereignty in practice, as states with greater political and military influence are better positioned to assert contested interpretations without meaningful challenge.
Selective Sovereignty in Practice: Self-Defense Without Consent
The contemporary practice of extraterritorial self-defense has developed primarily through state practice rather than judicial clarification, making empirical evidence central to evaluating its legality. Since 2014, multiple states have invoked Article 51 of the UN Charter to justify the use of force against non-state armed groups operating from another state’s territory without that state’s consent. These justifications reveal a consistent pattern in which sovereignty is treated not as a fixed legal constraint, but as a conditional principle subject to unilateral assessment.
In September 2014, the United States notified the Security Council that it was using force in Syria in collective self-defense of Iraq against ISIS, asserting that Syria was “unable or unwilling” to prevent its territory from being used for armed attacks. The letter framed necessity broadly, without articulating a detailed factual assessment of Syria’s capacity or willingness to address the threat. As Deeks observes, such reliance on unilateral necessity determinations risks transforming the “unwilling or unable” test from a restrictive condition into a permissive justification.
The United Kingdom adopted a similar legal position in November 2015, explicitly endorsing collective self-defense as the basis for airstrikes in Syria without Syrian consent. Turkey likewise invoked Article 51 to justify cross-border operations against non-state armed groups in Syria and Iraq, grounding its actions in claims of ongoing armed attacks and the territorial state’s failure to suppress them. Across these cases, sovereignty is not denied in principle but is rendered contingent on the acting state’s assessment of necessity.
This pattern reflects what Dapo Akande has described as an expansion in the operational interpretation of self-defense against non-state actors, particularly where territorial states are politically weak or diplomatically isolated. While the UN Charter does not distinguish between strong and weak states in its protection of territorial integrity, contemporary practice suggests that the practical force of sovereignty varies with power and legitimacy. As a result, similar uses of force by less powerful states invoking self-defense have often been treated as unlawful or destabilizing, reinforcing an asymmetry in the application of jus ad bellum constraints.
Selective sovereignty thus emerges not from doctrinal ambiguity alone, but from how legal justifications are invoked and received in practice. When powerful states can unilaterally determine unwillingness or inability with limited scrutiny, sovereignty ceases to operate as a reciprocal legal shield and instead becomes selectively enforceable. This development poses a structural challenge to the law-of-war framework, which depends on consistent application to maintain its constraining function.
From Self-Defense to Armed Conflict: Law-of-War Consequences
Extraterritorial uses of force justified as self-defense can quickly cross the threshold into a non-international armed conflict, triggering the application of international humanitarian law regardless of geographic boundaries. Once that threshold is met, the acting state’s operations are governed not only by jus ad bellum, but also by jus in bello obligations, including distinction, proportionality, and precautions in attack. Framing such operations as episodic or limited acts of self-defense risks obscuring these legal consequences and diluting compliance with the law of armed conflict.
Schmitt has emphasized that the existence of an armed conflict depends on the intensity of hostilities and organization of the parties, not on the legal labels states assign to their actions. In practice, sustained cross-border strikes against organized non-state armed groups frequently satisfy these criteria, even when states avoid formally acknowledging an armed conflict. This disconnect between legal characterization and operational reality creates uncertainty regarding the applicable legal framework and accountability mechanisms.
Selective sovereignty exacerbates this problem by shaping when and how the law of armed conflict is acknowledged. Powerful states invoking self-defense have often treated International Humanitarian Law (IHL) applicability as implicit or secondary, while emphasizing jus ad bellum justifications to legitimize operations. This approach risks normalizing a model in which territorial sovereignty is bypassed without a corresponding commitment to the full constraints imposed by IHL.
The DoD Law of War Manual underscores that compliance with IHL is mandatory whenever an armed conflict exists, irrespective of whether the use of force is justified under Article 51 of the UN Charter. Failure to clearly recognize the transition from self-defense to armed conflict may therefore undermine the protective function of IHL by allowing operational practices to develop without adequate legal scrutiny. When sovereignty is applied selectively at the jus ad bellum stage, the downstream effect is often selective engagement with jus in bello obligations as well.
This dynamic highlights a broader structural concern: selective sovereignty does not merely affect the legality of crossing borders, it also reshapes how and when the law of armed conflict is operationalized. As a result, legal constraints risk becoming fragmented across stages of analysis, weakening the coherence of the law-of-war framework as a whole.
Reciprocity, Precedent, and the Risk of Normalization
The law of armed conflict relies not only on formal rules, but on reciprocity and shared expectations regarding how those rules are applied. When states invoke self-defense to justify extraterritorial force without consent, the legal reasoning they advance does not operate in isolation, but contributes to a body of precedent that other actors may later rely upon. Over time, repeated practice risks normalizing interpretations of self-defense that treat territorial sovereignty as presumptively defeasible.
Selective sovereignty intensifies this dynamic by weakening the reciprocal character of jus ad bellum constraints. As the DoD Law of War Manual emphasizes, the credibility of the law governing the use of force depends on consistent application and good-faith adherence to necessity and proportionality requirements. When powerful states assert expansive self-defense claims with limited evidentiary burden, they lower the threshold for lawful force in ways that cannot easily be confined to their own conduct.
This risk is not hypothetical. Other states have already begun invoking similar reasoning to justify cross-border operations against non-state actors, often citing the same Article 51 language employed by Western states in earlier interventions. Once such arguments enter the legal vocabulary of international practice, they become available to a wider range of actors, including those with fewer incentives to exercise restraint.
From a law-of-war perspective, the danger lies less in any single invocation of self-defense than in the cumulative effect of selective legal reasoning. As Deeks has cautioned, doctrines developed to address exceptional security threats can harden into permissive norms when repeatedly asserted without rigorous scrutiny. This process risks transforming contested interpretations into assumed entitlements, eroding the constraining function of sovereignty over time.
Reciprocity also operates at the operational level. If sovereignty is treated as conditional during the jus ad bellum analysis, similar selectivity may emerge in the application of jus in bello obligations once hostilities commence. The result is a fragmented legal framework in which constraints vary not with the nature of the conflict, but with the identity of the actor invoking them. Such asymmetry undermines the law of armed conflict’s central purpose: to impose predictable and universal limits on the use of force, even under conditions of military necessity.
Operational and Legal Implications
The selective invocation of sovereignty in self-defense claims carries significant operational and strategic consequences. First, expanding interpretations of the unwilling or unable doctrine risks lowering the threshold for cross-border force, particularly in conflicts involving non-state actors. Although the doctrine has been articulated in state practice and defended by some scholars as consistent with Article 51 of the UN Charter, it remains contested in both scope and status. If powerful states treat territorial consent as optional when host states are deemed ineffective, weaker states may find their territorial integrity increasingly contingent upon external assessments of capacity, thereby straining the prohibition on the use of force under Article 2(4) of the Charter.
Second, such practice invites reciprocity. The International Court of Justice has repeatedly affirmed the centrality of the prohibition on the use of force and the restrictive nature of self-defense in cases such as Nicaragua v. United States and Armed Activities on the Territory of the Congo. If states adopt broad interpretations of necessity and imminence while resisting similar claims by others, the coherence and stability of the jus ad bellum framework risk gradual erosion. The normalization of extraterritorial force against non-state actors, particularly absent clear attribution, may weaken shared understandings of Article 2(4), especially in regions characterized by ongoing non-international armed conflicts.
Third, ambiguity surrounding sovereignty and self-defense complicates the relationship between jus ad bellum and jus in bello. While the law of armed conflict regulates conduct once hostilities begin, its application presupposes the existence of an armed conflict as defined in international humanitarian law. Contested cross-border uses of force may blur the legal thresholds that determine when an international or non-international armed conflict exists, thereby affecting targeting rules, detention authority, and civilian protection obligations. The interaction between the legality of force under Article 51 and the classification of conflict under IHL thus becomes operationally significant.
In this sense, selective sovereignty is not merely a theoretical concern. It shapes the legal environment in which military planners, government legal advisers, and policymakers assess cross-border operations, necessity determinations, and the risks of reciprocal legal claims.
Conclusion
The contemporary practice of extraterritorial self-defense against non-state armed groups reveals not the disappearance of sovereignty, but its selective application. As Article 51 has shown, sovereignty continues to function as a formal legal constraint within the jus ad bellum framework, yet its practical force varies depending on how doctrines such as necessity, consent, and unwillingness or inability are invoked and received in practice. Where these doctrines are treated as permissive entitlements rather than limiting conditions, sovereignty risks becoming conditional rather than reciprocal.
This selectivity carries consequences that extend beyond the initial legality of crossing borders. When self-defense claims are asserted without rigorous evidentiary scrutiny, they weaken the predictability of the law governing the use of force and complicate the transition to the law of armed conflict once hostilities reach the threshold of armed conflict. The result is not merely doctrinal ambiguity, but operational uncertainty regarding the scope and application of international humanitarian law obligations.
The challenge, however, is not resolved by rejecting extraterritorial self-defense outright. Contemporary security threats posed by organized non-state armed groups are real, and international law must remain capable of addressing them. What is required instead is discipline: a restrained understanding of “unwilling or unable” as a contextual argument within necessity, clear acknowledgment of when armed conflict exists, and consistent application of legal constraints regardless of the actor invoking them.
Ultimately, the credibility of the law of armed conflict depends less on formal doctrinal innovation than on the even application of existing rules. If sovereignty is to retain its constraining function, it must operate as a reciprocal legal principle rather than a selectively enforced privilege. Preserving that reciprocity is essential not only for legal coherence, but for the long-term legitimacy of the international legal order governing the use of force.
