The Evolution of Self-Defence Against Non-State Actors in International Law

I. Introduction: From Inter-State War to Transnational Threats

Self-Defence Against Non-State Actors sits at the fault line between the UN Charter’s state-centric security model and contemporary patterns of cross-border violence. The Charter prohibition on the use of force was drafted for a world where the paradigmatic threat was one State attacking another. Yet modern armed groups—terrorist networks, insurgent movements with transnational reach, and other non-state entities capable of large-scale violence—have repeatedly generated situations where a victim State claims that it must use force outside its territory to stop attacks that originate abroad. The resulting legal question is no longer marginal. It is central to how the jus ad bellum is interpreted, applied, and constrained today.

The problem is doctrinal as much as it is operational. Article 51 recognizes an “inherent” right of self-defence “if an armed attack occurs,” but it does not expressly identify the attacker. That silence has produced two competing legal narratives:

1. A restrictive narrative that treats self-defence as functionally inter-State: force is lawful only when the non-state violence is attributable to a State (through meaningful State involvement under the law of responsibility), or when the territorial State consents, or when the Security Council authorizes force.

   
   

2. A permissive narrative that treats the gravity of the attack as the key trigger: if a non-state actor launches an armed attack of sufficient scale and effects, self-defence can be invoked even absent attribution—provided necessity and proportionality are met, and the territorial State cannot or will not address the threat.

The International Court of Justice has historically pulled doctrine toward the restrictive narrative by emphasizing attribution logic and inter-State framing in its approach to “armed attack.” At the same time, post-2001 state practice has often moved in the opposite direction, increasingly asserting that non-state actors can trigger self-defence without attribution, especially where the territorial State is described as incapable of suppressing the threat. This divergence is not an academic detail; it is the reason the field remains unsettled and contested.

A second tension is structural: self-defence against a non-state actor operating abroad is also, unavoidably, a use of force on another State’s territory. Even if the defending State claims it is striking only the armed group, the operation engages the territorial State’s sovereignty and territorial integrity. In other words, the dispute is not only about who can trigger self-defence, but also about where self-defence may be lawfully exercised and under what conditions that exercise can avoid becoming an open-ended license for extraterritorial force.

This is where the legal debate becomes most fragile. If the law insists on attribution as a prerequisite, victim States may be left without a lawful response in scenarios where attribution is factually difficult, politically disputed, or intentionally obscured. If the law abandons attribution too easily, the constraint function of Article 2(4) risks dilution: “self-defence” can become a rhetorical label attached to interventions that are not genuinely necessary, not proportionate, or not directed at stopping an armed attack. Both paths create systemic costs—either by making the Charter framework seem unrealistic, or by making it permissive in ways that undermine stability.

Accordingly, the aim of this article is precise: to trace the evolution of the legal doctrine of Self-Defence Against Non-State Actors and to clarify the legal tests that currently shape (and limit) extraterritorial defensive force. The article does not assume that practice automatically becomes law, nor that judicial statements alone can freeze doctrine. Instead, it examines the interaction among: (i) Charter text and structure, (ii) customary law principles—especially necessity and proportionality, (iii) the ICJ’s interpretive influence, and (iv) articulated state positions that claim to apply Article 51 to non-state violence.

Three guiding questions organize the analysis that follows:

• Trigger: When can violence by a non-state actor qualify as an “armed attack” for the purposes of Article 51?

• Attribution and sovereignty: When, if ever, is attribution to a State legally required, and what role do consent and territorial sovereignty play in lawful action?

• Constraints: Which limiting principles realistically prevent abuse—necessity, proportionality, temporal proximity, evidentiary standards, and (where invoked) the “unwilling or unable” logic?

The core claim tested throughout is straightforward: the evolution of Self-Defence Against Non-State Actors is best understood as a struggle to keep Article 51 operational without collapsing Article 2(4)’s constraint. The doctrine’s credibility depends on whether it can produce legally disciplined answers in hard cases—cases where the threat is real, the attacker is not a State, the territorial State is not consenting, and the Security Council is not providing decisive authorization.

II. The Classical Architecture of Self-Defence in International Law

The contemporary debate on self-defence cannot be understood without first clarifying the classical legal architecture that governs the use of force. This framework emerged through a gradual consolidation of customary rules and treaty law, culminating in the United Nations Charter. Its central function is restrictive: to limit unilateral resort to force while preserving a narrow exception for genuine defensive necessity.

1. Pre-Charter Foundations and Customary Law

Before 1945, international law did not contain a comprehensive prohibition on the use of force. War was considered a lawful instrument of state policy, subject primarily to political judgment rather than legal authorization. Within that permissive environment, self-defence developed as a justificatory concept rather than an exception to a general ban.

The most enduring doctrinal contribution from this period is the formulation associated with the Caroline incident, which articulated three cumulative requirements for lawful defensive force: necessity, immediacy, and proportionality. Necessity required that force be the only available means to repel the threat; immediacy demanded temporal proximity between the threat and the response; proportionality limited the scale and scope of defensive measures to what was required to neutralize the danger. These principles did not depend on treaty law and are widely regarded as crystallized customary rules that survived the transition to the Charter system.

2. The UN Charter and the Prohibition of Force

The adoption of the UN Charter marked a structural transformation of the law on the use of force. Article 2(4) established a general prohibition on the threat or use of force against the territorial integrity or political independence of any State. This provision reoriented international law away from discretionary warfare and toward collective security.

Self-defence was preserved, but recast. Article 51 recognizes an “inherent right” of individual and collective self-defence, confirming that the Charter did not create the right but constrained its exercise. The provision permits defensive force only if an armed attack occurs and only until the Security Council has taken measures necessary to maintain international peace and security. The right is therefore exceptional, temporary, and subject to institutional oversight.

This architecture reflects a deliberate balance. The Charter aims to prevent unilateral force as a default, while allowing narrowly tailored defensive action in emergency situations. Self-defence is not framed as a competing principle to the prohibition of force but as its most limited exception.

3. The Armed Attack Threshold

Central to the Charter framework is the concept of “armed attack.” Not every use of force qualifies. The armed attack threshold serves as a gatekeeping device, distinguishing grave violations that justify defensive force from lesser incidents that must be addressed through non-forcible means.

Classically, armed attack was understood in inter-State terms: military action by one State’s regular forces against another State’s territory, population, or armed forces. This understanding was reinforced by early judicial interpretations that emphasized the scale and effects of the violence rather than isolated or indirect acts.

The armed attack requirement is therefore both qualitative and quantitative. It limits self-defence to situations of serious harm and functions as a safeguard against escalation. Within the classical architecture, the threshold was designed to prevent States from invoking self-defence in response to minor frontier incidents, economic coercion, or political interference.

4. Necessity and Proportionality as Continuing Constraints

Although Article 51 does not expressly restate the Caroline criteria, necessity and proportionality remain integral to the law of self-defence. They operate as substantive constraints that apply regardless of the identity of the attacker or the location of the defensive action.

Necessity requires that defensive force respond to a genuine threat and that non-forcible alternatives be inadequate. Proportionality requires that the response be limited to what is required to repel or prevent the armed attack, without pursuing broader strategic or punitive objectives.

These principles perform a crucial stabilizing function. They ensure that self-defence remains reactive rather than opportunistic and that the exception does not swallow the rule. In classical doctrine, failure to satisfy necessity or proportionality renders a self-defence claim unlawful even when an armed attack has occurred.

5. Collective Self-Defence and Institutional Oversight

Article 51 also recognizes collective self-defence, allowing States to assist a victim State at its request. This mechanism was designed to accommodate regional security arrangements while maintaining the primacy of the Security Council.

Two procedural elements are essential in this context. First, the victim State must declare that it has been subjected to an armed attack and request assistance. Second, measures taken in self-defence must be reported to the Security Council. These requirements reinforce the exceptional nature of defensive force and preserve the Council’s supervisory role.

Within the classical architecture, collective self-defence does not expand the substantive scope of self-defence. It merely permits shared execution of an otherwise lawful defensive response.

6. Structural Assumptions of the Classical Model

The classical framework rests on implicit assumptions that shaped its design. It presumes that the principal threats to peace are inter-State, that armed attacks are attributable to States, and that territorial sovereignty is the primary unit of legal protection. Under these assumptions, attribution is relatively straightforward, consent is clearly identifiable, and the distinction between internal and international conflict is stable.

These assumptions explain why the Charter framework is precise in some areas and silent in others. The absence of explicit reference to non-state attackers reflects the historical context rather than a conscious exclusion. The classical architecture was not built to address persistent transnational armed violence by actors operating independently of State command structures.

This structural mismatch sets the stage for the modern controversy. As later sections will demonstrate, attempts to apply this classical architecture to non-state actors expose doctrinal gaps and interpretive tensions, particularly concerning attribution, sovereignty, and the limits of unilateral defensive force.

III. Armed Attack and the Identity of the Attacker

The concept of “armed attack” is the doctrinal hinge on which the law of self-defence turns. It determines not only when force may be used, but also against whom. In the classical framework, the identity of the attacker was implicitly assumed to be a State. Contemporary practice has disrupted that assumption, forcing international law to confront a question that Article 51 leaves unanswered: does the right of self-defence depend on who commits the attack, or on the nature and gravity of the attack itself?

1. Textual Silence and Its Legal Significance

Article 51 authorizes self-defence “if an armed attack occurs,” without specifying the legal status of the attacker. This textual silence is legally consequential. It suggests that the trigger for self-defence is the occurrence of an armed attack as a factual and legal event, not the formal classification of the entity that carried it out.

In treaty interpretation, silence does not automatically imply exclusion. The absence of limiting language supports the view that the Charter preserved a pre-existing customary right whose scope was not exhaustively codified. The reference to the “inherent” right of self-defence reinforces this understanding, indicating continuity rather than replacement.

At the same time, silence creates interpretive risk. Without explicit guidance, courts and States have filled the gap in divergent ways, often importing assumptions from a state-centric legal order that was dominant at the time of drafting.

2. The Armed Attack Threshold as a Functional Test

The armed attack requirement has always functioned as a gravity threshold. Not every use of force qualifies. The distinction between the most serious forms of force and lesser uses serves to limit escalation and preserve the prohibition on force as the rule.

Under this functional approach, the decisive factor is the scale and effects of the violence. Large-scale attacks causing significant loss of life, destruction of infrastructure, or sustained military engagement fall within the core meaning of armed attack. Isolated incidents, sporadic border clashes, or indirect support typically do not.

If the armed attack threshold is understood in functional terms, the identity of the attacker becomes secondary. A large-scale attack carried out by a non-state actor can, in material terms, produce consequences indistinguishable from those caused by regular armed forces. This analytical move underpins arguments that self-defence should not be denied solely because the attacker lacks statehood.

3. Judicial Emphasis on Attribution

Despite the textual and functional arguments, international judicial practice has often linked armed attack to attribution. In its jurisprudence, the International Court of Justice has emphasized that armed attacks are, in principle, acts imputable to States, and that support to non-state groups short of direct involvement does not automatically qualify.

This approach reflects caution rather than textual necessity. By anchoring armed attack to state responsibility, the Court reinforces the inter-State character of the Charter system and protects territorial sovereignty. Attribution operates as a filter: only violence that can be legally connected to a State may justify force against that State’s territory.

The consequence, however, is that non-state actors capable of independent and devastating violence occupy an ambiguous space. Their attacks may be acknowledged as serious and unlawful, yet insufficient to trigger self-defence unless a State link can be demonstrated. This creates a doctrinal asymmetry between factual harm and legal response.

4. Accumulation of Events and Non-State Violence

One attempt to reconcile gravity with attribution is the “accumulation of events” theory. Under this approach, a series of smaller attacks may collectively reach the armed attack threshold. This theory has been invoked in contexts involving non-state actors who engage in sustained cross-border violence rather than single catastrophic strikes.

While the accumulation concept addresses scale over time, it does not resolve the identity problem. Courts and commentators remain divided on whether repeated non-state attacks can, by themselves, amount to an armed attack triggering self-defence, or whether attribution to a State remains a necessary legal bridge.

The theory thus mitigates, but does not eliminate, the tension between gravity-based reasoning and attribution-based doctrine.

5. Post-2001 Reorientation of State Practice

State practice since the early twenty-first century has increasingly emphasized the consequences of attacks rather than the formal status of attackers. States responding to transnational terrorism have argued that non-state actors can launch armed attacks of sufficient gravity to activate self-defence, especially when those actors control territory, possess heavy weaponry, or conduct coordinated operations over time.

This practice reflects a pragmatic assessment of threat rather than a doctrinal rejection of sovereignty. The argument advanced is not that attribution is irrelevant, but that attribution is not the only route to legality. Where a non-state actor’s violence reaches the armed attack threshold and defensive force is necessary and proportionate, the absence of direct State involvement should not, by itself, bar self-defence.

6. Normative Risks of Identity-Based and Consequence-Based Approaches

Both approaches carry legal risks. An identity-based model, tied rigidly to attribution, risks rendering the law unresponsive to real security threats posed by autonomous non-state actors. It may also incentivize States to conceal involvement or operate through proxies to avoid legal consequences.

A purely consequence-based model, detached from attribution and sovereignty concerns, risks normalizing cross-border force whenever a State asserts that it faces a serious threat. Without clear limits, self-defence could become a flexible justification for intervention rather than an emergency exception.

The unresolved question, therefore, is not simply whether non-state actors can commit armed attacks, but how international law can recognize that possibility while maintaining meaningful constraints on the use of force.

7. Doctrinal Implications

The debate over armed attack and attacker identity exposes a structural tension in the jus ad bellum. The law must choose between preserving a strictly inter-State conception of self-defence and adapting its trigger conditions to reflect contemporary forms of violence. Neither choice is cost-free.

This tension explains why subsequent debates have shifted toward questions of territorial sovereignty, consent, and necessity. If identity alone cannot resolve the legality of self-defence against non-state actors, other doctrinal tools must shoulder the burden of restraint. The next sections examine how attribution, state responsibility, and consent have been deployed to manage this unresolved core problem.

IV. Non-State Actors as Sources of Armed Attacks

The question of whether non-state actors can constitute independent sources of armed attacks lies at the heart of the contemporary debate on self-defence. This issue is not merely semantic. It determines whether international law can respond coherently to transnational violence that is neither incidental nor marginal, but sustained, organized, and strategically directed against States.

1. Conceptualizing Non-State Armed Violence

Non-state actors are not a homogeneous category. They include terrorist organizations, insurgent movements, transnational armed groups, and hybrid entities exercising varying degrees of territorial control and governance functions. What distinguishes the actors relevant to the law of self-defence is not their political motivation or internal structure, but their capacity to conduct violence across borders at a level comparable to State military operations.

Historically, international law treated non-state violence as internal disturbance, criminal activity, or insurgency falling within domestic jurisdiction. That classification presupposed limited scale, localized impact, and the absence of strategic military capability. Contemporary non-state actors increasingly defy those assumptions.

2. Scale, Organization, and Operational Capability

The defining characteristic that pushes non-state violence into the realm of armed attack is scale and effects, not formal status. Modern non-state actors have demonstrated the ability to plan, coordinate, and execute operations involving mass casualties, prolonged campaigns, and control of territory. These actions are often indistinguishable in impact from attacks carried out by regular armed forces.

Three factors are particularly relevant in assessing whether non-state violence qualifies as an armed attack:

• Organizational structure: sustained command-and-control capacity, training systems, and logistical networks.

• Operational continuity: repeated or ongoing attacks rather than isolated incidents.

• Consequential impact: significant loss of life, destruction of infrastructure, or severe disruption of State functions.

When these elements are present, the factual reality challenges any rigid insistence that only States can generate armed attacks.

3. Armed Attack Beyond the State Paradigm

The classical inter-State paradigm assumes that armed attacks are manifestations of sovereign decision-making. This assumption becomes unstable when non-state actors operate autonomously, finance themselves independently, and pursue strategic objectives without direct State direction.

In such cases, insisting on attribution as a prerequisite for recognizing an armed attack conflates two distinct legal inquiries:

1. Whether an armed attack has occurred (a factual and threshold-based question).

2. Who bears responsibility for the attack (a question of attribution and State responsibility).

Treating attribution as determinative of the existence of an armed attack collapses these inquiries into one. This approach risks denying legal recognition to attacks that are materially severe simply because responsibility cannot be legally imputed to a State.

4. Accumulation and Persistence of Non-State Attacks

Non-state actors rarely operate through a single decisive strike. Instead, they tend to engage in persistent patterns of violence, testing defenses, exploiting territorial sanctuaries, and escalating over time. The cumulative impact of such conduct may reach or exceed the armed attack threshold even if individual incidents, viewed in isolation, do not.

The accumulation of events approach is particularly relevant here. Sustained cross-border attacks by non-state actors can generate a security situation equivalent to that produced by conventional military campaigns. The legal difficulty lies not in recognizing the gravity of such situations, but in articulating criteria that prevent overextension of self-defence claims.

5. Territorial Control and Quasi-State Functions

A further complication arises when non-state actors exercise effective control over territory. In these circumstances, the distinction between State and non-state violence becomes operationally blurred. Groups that control territory may train fighters, levy resources, administer populations, and launch attacks with relative impunity.

While territorial control does not confer statehood, it does affect the legal analysis. The ability to use territory as a platform for sustained armed operations strengthens the argument that such actors can generate armed attacks within the meaning of self-defence doctrine. At the same time, it intensifies the sovereignty dilemma, as defensive force necessarily occurs on territory claimed by a State.

6. The Risk of Doctrinal Inflation

Recognizing non-state actors as sources of armed attacks carries legal risks. If the threshold is set too low, States may characterize a wide range of security threats as armed attacks, weakening the prohibition on force. Criminal violence, sporadic border incidents, or internal unrest could be reframed as international armed attacks, expanding the scope of unilateral force.

This risk underscores the importance of strict threshold criteria. Only non-state violence that reaches a high level of organization, intensity, and transboundary impact should be capable of triggering self-defence. The doctrine cannot rely on political labeling or rhetorical urgency; it must be anchored in objective assessment.

7. Implications for the Law of Self-Defence

The recognition of non-state actors as potential sources of armed attacks does not automatically validate defensive force. It merely opens the door to further legal scrutiny. Necessity, proportionality, temporal proximity, and respect for territorial sovereignty remain indispensable constraints.

This section establishes a foundational point for the remainder of the analysis: non-state actors can, in principle, generate armed attacks within the meaning of self-defence law, but that recognition intensifies the need for disciplined legal standards.

The next stage of the inquiry addresses how attribution, State responsibility, and the conduct of the territorial State shape the legality of defensive responses to such attacks.

V. Attribution, State Involvement, and the Limits of Responsibility

Attribution occupies a pivotal position in the law governing self-defence against non-state actors. It operates at the intersection of two legal regimes: the jus ad bellum, which regulates the lawful use of force, and the law of State responsibility, which determines when conduct can be legally imputed to a State. Confusion between these regimes has significantly shaped the debate on the permissibility of defensive force against non-state actors.

1. Attribution as a Conceptual Bridge

In classical international law, attribution served a relatively straightforward function. Armed attacks were presumed to be acts of States, carried out by regular armed forces or by entities acting under State direction or control. Attribution, therefore, aligned the factual occurrence of violence with legal responsibility.

In the context of non-state actors, attribution becomes a doctrinal bridge rather than a descriptive assumption. It is used to justify treating non-state violence as legally equivalent to State action, thereby permitting defensive force against the territorial State or within its territory. This move transforms attribution from a responsibility tool into a gatekeeper for self-defence.

2. Standards of State Involvement

International law recognizes multiple standards for attributing conduct of non-state actors to States. These standards are demanding by design and reflect the protective function of sovereignty.

The most stringent standard requires effective control, meaning that the State directed or controlled the specific operation in which the wrongful act occurred. A lower threshold of overall control, sometimes discussed in the context of organized armed groups, focuses on broader organizational control rather than specific acts. Additional forms of involvement include financing, training, equipping, or providing intelligence and safe haven.

Within the law of State responsibility, not all forms of support are sufficient for attribution. Assistance, tolerance, or failure to prevent harmful conduct generally does not convert non-state acts into State acts. This high bar is intentional; it prevents automatic liability and preserves the distinction between complicity and direct responsibility.

3. Attribution and the Armed Attack Requirement

The central doctrinal controversy arises when attribution standards are imported into the armed attack analysis. When attribution is treated as a prerequisite for recognizing an armed attack, the existence of the attack itself becomes contingent on proof of State involvement.

This approach creates a structural tension. The gravity of the violence may be undisputed, yet the legal capacity to respond in self-defence depends on evidentiary thresholds that are often difficult to meet in real time. Covert support, proxy relationships, and deniability strategies further complicate attribution, particularly in transnational terrorism contexts.

As a result, the attribution requirement can function less as a safeguard and more as a disabling rule, preventing lawful defensive action even in the face of serious harm.

4. Non-Attributable Violence and Legal Gaps

Situations in which non-state actors operate independently, or with minimal and indirect State involvement, expose the limits of attribution-based reasoning. In such cases, there may be:

• Clear evidence of sustained cross-border attacks;

• A territorial State that lacks effective control over the relevant area;

• No legally sufficient basis to attribute the conduct to the State under responsibility rules.

If attribution is treated as indispensable, these scenarios generate a legal gap. The victim State faces ongoing armed violence but lacks a lawful avenue for self-defence that involves force beyond its borders. This gap has been one of the primary drivers behind the evolution of alternative justificatory frameworks.

5. Distinguishing Responsibility from Permission

A critical analytical step is separating responsibility from permission. Attribution answers the question of who is legally responsible for a wrongful act. Self-defence addresses whether a State is permitted to use force in response to an armed attack.

Conflating these inquiries leads to conceptual distortion. A State may be entitled to defend itself against an armed attack without assigning legal responsibility to another State. Conversely, attribution of responsibility does not automatically authorize defensive force if necessity or proportionality is absent.

Recognizing this distinction allows for a more nuanced approach. It permits the law to acknowledge non-state armed attacks as triggering self-defence while maintaining strict limits on when force may be used against or within another State’s territory.

6. The Limits of State Responsibility as a Regulatory Tool

The law of State responsibility was not designed to regulate the immediate use of force in response to security threats. Its evidentiary standards, temporal horizons, and remedial focus differ from the urgent and preventive logic of self-defence.

Relying exclusively on attribution to govern self-defence risks misaligning legal tools with practical realities. Attribution remains relevant for determining international wrongfulness and secondary obligations, but it cannot bear the entire regulatory burden of the jus ad bellum in cases involving autonomous non-state actors.

7. Doctrinal Consequences

The limits of attribution have reshaped the legal landscape. As States increasingly confront non-attributable armed violence, they have sought alternative justifications for extraterritorial defensive force that do not depend on proving State responsibility.

This development does not render attribution irrelevant. State involvement continues to strengthen self-defence claims and may affect the scope of permissible action. However, attribution is no longer the sole axis around which legality turns.

The next stage of analysis, therefore, moves beyond responsibility to examine consent, territorial sovereignty, and necessity-based frameworks that have emerged to manage situations where attribution fails but the threat persists.

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VI. Consent of the Territorial State and Sovereignty Constraints

Consent of the territorial State has long been recognized as a lawful basis for the presence or use of force by another State on its territory. In the context of self-defence against non-state actors, consent functions as a critical sovereignty-preserving mechanism, allowing defensive action without triggering the prohibition on the use of force. At the same time, the limits of consent expose structural tensions between territorial integrity and the security needs of the victim State.

1. Consent as a Circumstance Precluding Wrongfulness

In international law, valid consent by a State precludes the wrongfulness of conduct that would otherwise violate an international obligation. Applied to the use of force, consent permits foreign military operations on a State’s territory without breaching the prohibition in Article 2(4). Where consent is present, the legal analysis shifts away from self-defence and toward the scope and conditions of the consent itself.

Consent must be attributable to the competent authorities of the territorial State and must be freely given. It can be limited in time, geography, and purpose, and it may be withdrawn. Defensive operations that exceed the terms of consent risk becoming unlawful even if the initial presence was permitted.

2. Forms of Consent in Practice

Consent in self-defence contexts appears in several forms. Express consent is the clearest, typically articulated through diplomatic communication or formal requests for assistance. Ad hoc consent may be granted in response to specific incidents or operations. In some cases, continuing consent is inferred from ongoing cooperation between States.

More controversial is the notion of implicit or tacit consent, inferred from acquiescence or failure to object. While silence may carry legal weight in some areas of international law, reliance on tacit consent in the use of force context is precarious. Given the gravity of sovereignty interests at stake, consent to foreign military action is generally expected to be clear and demonstrable.

3. Consent and Governmental Authority

The validity of consent depends on the status of the consenting authority. Consent must emanate from the internationally recognized government of the territorial State. Internal political divisions, contested governments, or parallel authorities complicate the analysis but do not automatically invalidate consent.

A government need not exercise effective control over its entire territory to issue a valid consent. However, the gap between formal authority and effective control can undermine the practical value of consent. A State may consent to foreign action yet lack the capacity to ensure that operations remain confined to agreed parameters.

4. Failed States and Capacity Constraints

Consent becomes problematic in situations involving failed or severely weakened States. Where governmental institutions are unable to exercise authority or control territory, the formal ability to grant consent may exist alongside factual incapacity to suppress non-state actors.

In such cases, two competing considerations arise. On one hand, respect for sovereignty suggests that consent remains the preferred legal basis for action. On the other hand, reliance on consent alone may be insufficient when the territorial State cannot meaningfully address the threat or regulate foreign operations. This tension has contributed to arguments that defensive force may be permissible even absent effective consent, provided strict necessity is demonstrated.

5. Sovereignty as Both Right and Responsibility

Territorial sovereignty is not merely a right to exclude foreign force; it also entails a responsibility to prevent a State’s territory from being used as a platform for attacks against other States. This dual character of sovereignty has gained prominence in debates on self-defence against non-state actors.

Where a territorial State actively suppresses non-state threats or cooperates with the victim State, its sovereignty interests are reinforced. Where it is unwilling or unable to act, the claim that sovereignty bars all external defensive measures becomes more contested. The legal challenge lies in translating this responsibility concept into concrete and objective criteria.

6. The Limits of Consent as a Regulatory Tool

Consent cannot serve as a universal solution. It may be withheld for political reasons unrelated to capacity or good faith. It may be withdrawn abruptly. It may also be granted covertly, creating evidentiary uncertainty. Overreliance on consent risks allowing the legality of self-defence to hinge on diplomatic alignment rather than legal principle.

Moreover, treating consent as the sole lawful route to extraterritorial defensive force risks incentivizing territorial States to withhold cooperation while continuing to tolerate non-state armed activity. Such outcomes undermine the preventive function of the jus ad bellum.

7. Implications for Sovereignty Constraints

The role of consent highlights the broader sovereignty dilemma at the core of self-defence against non-state actors. Sovereignty remains a foundational constraint, but it is no longer absolute in contexts of persistent transnational armed violence.

The law, therefore, faces a balancing exercise. It must preserve territorial integrity as a default rule while recognizing that sovereignty cannot be used as a shield for inaction in the face of serious cross-border harm. This balance sets the stage for the emergence of necessity-based doctrines, particularly those that attempt to operationalize when the absence of consent may be legally overcome.

VII. The “Unwilling or Unable” Standard

The “unwilling or unable” standard has emerged as one of the most controversial attempts to reconcile territorial sovereignty with the perceived necessity of defensive force against non-state actors. It is not codified in treaty law, nor has it been authoritatively endorsed in general terms by international courts. Its significance lies instead in its function: it seeks to operationalize necessity when defensive action is contemplated on the territory of a non-consenting State.

1. Conceptual Origins and Rationale

The standard is rooted in a practical dilemma. When a non-state actor launches or prepares armed attacks from within a State’s territory, and the territorial State does not consent to foreign action, the victim State faces two competing legal imperatives. One is the obligation to respect sovereignty and territorial integrity. The other is the right to defend itself against serious harm.

The “unwilling or unable” logic attempts to mediate this conflict by shifting focus from formal consent to effective threat suppression. If the territorial State is unwilling to address the threat, or objectively unable to do so, continued reliance on its territorial integrity as an absolute barrier is claimed to be unjustified.

2. Structure of the Standard

Although formulations vary, the standard typically rests on three cumulative inquiries:

1. Existence of a qualifying armed threat: The non-state actor must pose a real and ongoing danger that meets the armed attack threshold or is imminently capable of doing so.

2. Failure of territorial State action: The territorial State must be unwilling or unable to prevent the non-state actor’s operations in a meaningful way.

3. Necessity of external force: Defensive action by the victim State must be necessary and limited to addressing the threat, not broader strategic objectives.

This structure mirrors the traditional necessity test, but relocates it from inter-State confrontation to transnational threat management.

3. Unwillingness and Inability as Distinct Conditions

Unwillingness and inability are analytically distinct, though often conflated in practice.

• Unwillingness refers to situations where the territorial State has the capacity to act but chooses not to, tolerates the non-state actor, or selectively enforces its authority for political or strategic reasons.

• Inability refers to genuine capacity deficits, including a lack of effective control over territory, institutional collapse, or the inability to confront heavily armed groups.

Distinguishing these conditions matters. Claims of unwillingness require assessment of intent and conduct, while claims of inability require assessment of objective capacity. Both raise evidentiary challenges and demand careful substantiation.

4. Evidentiary and Procedural Challenges

A central weakness of the standard lies in its indeterminacy. There is no agreed-upon evidentiary threshold for establishing unwillingness or inability. States invoking the standard often rely on intelligence assessments that are not publicly verifiable, making external evaluation difficult.

This uncertainty creates asymmetry. The victim State is effectively positioned as both claimant and judge of the territorial State’s performance. Without procedural safeguards or external validation, the standard risks sliding into unilateral discretion.

5. Relationship with Sovereignty and Consent

The standard does not reject sovereignty outright. Instead, it reframes sovereignty as conditional on the territorial State’s performance of basic security functions. This reframing is controversial because it introduces functional criteria into a domain traditionally governed by formal equality and territorial control.

Critics argue that this move undermines the stability of the Charter system by weakening the default prohibition on force. Proponents counter that sovereignty cannot operate as a legal shield for persistent inaction when serious cross-border harm is foreseeable and preventable.

6. Risk of Abuse and Doctrinal Inflation

The most serious concern surrounding the “unwilling or unable” standard is its susceptibility to abuse. If broadly framed, it allows States to characterize a wide range of security situations as failures of territorial control, thereby justifying force without consent.

The absence of clear limits risks transforming an exceptional doctrine into a routine justification. Without strict linkage to necessity, proportionality, and immediacy, the standard may erode the distinction between self-defence and preventive intervention.

7. Normative Status and Legal Uncertainty

At present, the “unwilling or unable” standard occupies an ambiguous normative space. It reflects a pattern of asserted practice by some States, but lacks consistent articulation, judicial endorsement, or universal acceptance. As such, it cannot be treated as settled customary law.

Its significance lies less in its legal authority than in its explanatory power. It illustrates how States seek to adapt existing self-defence principles to situations involving non-state actors and non-consenting territorial States. The standard’s future depends on whether it can be disciplined through clearer criteria or incorporated into broader interpretive consensus.

8. Transition to Limiting Principles

The unresolved status of the “unwilling or unable” standard underscores the importance of traditional constraints. If the law is to accommodate defensive force against non-state actors without collapsing into permissiveness, necessity and proportionality must perform genuine limiting work.

The next section, therefore, returns to these core principles, examining how they function in contemporary self-defence claims and whether they can provide the restraint that attribution and consent increasingly fail to supply.

VIII. Necessity, Proportionality, and Temporal Constraints

As the law of self-defence confronts scenarios involving non-state actors, classical limiting principles assume heightened importance. Necessity, proportionality, and temporal constraints are no longer auxiliary considerations; they are the primary mechanisms through which the jus ad bellum seeks to prevent expansion of unilateral force. When attribution is uncertain and consent absent, these principles carry the full weight of legal restraint.

1. Necessity as the Core Justification

Necessity is the foundational requirement of self-defence. It demands that defensive force be a last resort, employed only when non-forcible measures are inadequate to address the threat. In the context of non-state actors, necessity requires more than the existence of hostility or generalized insecurity. It requires a demonstrable link between the threat posed by the actor and the need for immediate defensive action.

This inquiry is context-sensitive. Where the territorial State is actively suppressing the threat or willing to cooperate, the necessity for unilateral action diminishes. Conversely, where the threat is imminent or ongoing, and the territorial State is unable or unwilling to intervene effectively, the argument for necessity strengthens. The principle thus operates as a factual assessment, not a discretionary label.

2. Proportionality as a Limiting Function

Proportionality constrains the scale, scope, and intensity of defensive force. It does not require symmetry in weapons or casualties; it requires that force be directed solely toward neutralizing the armed threat.

In self-defence against non-state actors, proportionality prohibits expansive military campaigns aimed at regime change, territorial control, or punishment. Defensive force must be confined to what is reasonably required to halt or prevent the armed attack. Actions that exceed this objective, even if initially justified by self-defence, risk transforming lawful defense into unlawful use of force.

Proportionality also applies cumulatively. Even if individual strikes appear limited, a prolonged pattern of force may, taken as a whole, exceed what is proportionate to the original threat.

3. Temporal Constraints and Immediacy

Temporal proximity between the threat and the defensive response has long been a defining element of self-defence. Classical doctrine emphasized immediacy, requiring that force be used only while the threat was imminent or ongoing.

Non-state actors challenge this model. Their operations are often intermittent, involving planning cycles, pauses, and relocations rather than continuous confrontation. Strict immediacy, understood as temporal simultaneity, is therefore ill-suited to contemporary threats.

Modern analysis focuses instead on continuing or imminent threats. Defensive force may be temporally justified when it responds to a persistent campaign of attacks or to credible intelligence indicating an impending strike. However, temporal flexibility must not become temporal license. Past attacks alone cannot justify indefinite future force.

4. Anticipatory and Preventive Dimensions

The distinction between anticipatory and preventive action is critical. Anticipatory self-defence refers to force used in response to an imminent attack. Preventive action targets speculative or long-term threats.

In cases involving non-state actors, anticipatory self-defence is often claimed on the basis that waiting for an attack would negate the possibility of effective defense. This claim is strongest where intelligence is specific and reliable and where delay would significantly increase harm.

Preventive self-defence, by contrast, lacks broad legal acceptance. Force used against non-state actors based on generalized risk or strategic advantage undermines the requirement of necessity and threatens to erode the prohibition on force.

5. Evidentiary Standards and Good Faith Assessment

Necessity, proportionality, and temporal justification depend on credible evidence. Assertions of threat must be grounded in verifiable information, even if some details cannot be publicly disclosed. Good faith assessment is essential; self-defence cannot be justified retroactively by outcomes alone.

The burden of justification lies with the acting State. Failure to articulate the factual basis for necessity or to explain the proportionality of the response weakens the legal claim and invites skepticism.

6. Interaction with the “Unwilling or Unable” Standard

Where the “unwilling or unable” logic is invoked, necessity and proportionality must perform heightened scrutiny. The inability or unwillingness of the territorial State does not automatically justify force; it merely removes one obstacle. Defensive action must still be necessary to address the threat and proportionate to that purpose.

This interaction reinforces the idea that necessity-based doctrines do not replace classical limits but depend on them for legitimacy.

7. Restraining Function in Contemporary Practice

In contemporary self-defence claims against non-state actors, necessity, proportionality, and temporal constraints serve as the principal safeguards against doctrinal inflation. They are the criteria by which claims can be assessed, challenged, and evaluated.

If these principles are applied rigorously, self-defence remains an emergency response, not a policy tool. If applied loosely, they become rhetorical devices that mask expansive uses of force.

The next section examines how these limiting principles interact with collective security mechanisms and the role of institutional oversight, particularly the position of the Security Council in shaping or constraining unilateral defensive action.

IX. Security Council Practice and Collective Responses

The UN Security Council occupies a central position in the Charter system governing the use of force. Its primary responsibility for the maintenance of international peace and security frames self-defence as a provisional and exceptional measure, intended to operate only until collective mechanisms are activated. In debates on self-defence against non-state actors, Security Council practice plays a critical role in shaping, constraining, and indirectly legitimizing State action, even when it does not provide explicit authorization.

1. The Security Council’s Institutional Role in the Charter Framework

Article 51 situates self-defence within a system of collective security. Defensive measures are permitted only “until the Security Council has taken measures necessary” to address the threat. This formulation reflects a clear hierarchy: unilateral force is tolerated temporarily, but the Council retains ultimate authority to determine how peace and security should be restored.

In classical terms, this design assumes that the Council will respond decisively to armed attacks. In practice, political divisions, veto dynamics, and strategic interests often limit the Council’s ability to act promptly or coherently. This institutional reality has had significant consequences for the evolution of self-defence doctrine.

2. Post-2001 Council Practice and Non-State Threats

The Security Council’s response to large-scale terrorist violence in the early twenty-first century marked a turning point. The Council explicitly characterized transnational terrorism as a threat to international peace and security and reaffirmed the inherent right of self-defence in that context. Importantly, these pronouncements did not condition self-defence on attribution to a State, nor did they frame the threat exclusively in inter-State terms.

This practice has been widely interpreted as implicit recognition that non-state actors can generate situations capable of triggering self-defence. At the same time, the Council avoided articulating precise legal tests or thresholds, leaving States to operationalize the doctrine through their own interpretations.

3. Silence, Acquiescence, and Legal Significance

In many instances involving defensive force against non-state actors, the Security Council has neither authorized nor condemned the action. This silence has been invoked by some States as evidence of acquiescence or tacit acceptance.

Legally, silence is ambiguous. It does not amount to authorization, nor does it necessarily signal approval. However, persistent Council inaction in the face of repeated claims of self-defence can influence perceptions of legality, particularly when actions are reported under Article 51 and not met with formal objection.

This dynamic contributes to gradual normative drift. The absence of clear institutional guidance allows practice to develop unevenly, shaped more by political tolerance than by articulated legal standards.

4. Reporting Obligations and Transparency

Article 51 requires States exercising self-defence to report measures taken to the Security Council. This procedural obligation serves multiple functions: it promotes transparency, enables Council oversight, and provides a forum for legal and political scrutiny.

In the context of non-state actors, reporting has become an important site of legal argumentation. States often use reports to articulate their interpretation of self-defence, necessity, and territorial constraints. While reporting does not validate legality, failure to report undermines the credibility of a self-defence claim and weakens the connection to the collective security system.

5. Collective Self-Defence and Coalition Responses

Security Council practice also intersects with collective self-defence. States responding collectively to non-state threats often frame their actions as assistance to a victim State that has declared itself under armed attack. This framing reinforces procedural legitimacy and situates the response within recognized Charter mechanisms.

However, collective action does not eliminate legal constraints. The same requirements of necessity, proportionality, and respect for sovereignty apply. Coalition size or political support cannot substitute for legal justification.

6. Authorization Versus Endorsement

A key distinction in Security Council practice is between authorization and endorsement. Authorization provides a clear legal basis for force under the Charter. Endorsement, by contrast, may take the form of political support or recognition of a threat without conferring explicit legal authority.

In cases involving non-state actors, the Council has often opted for endorsement rather than authorization. This approach allows flexibility but sacrifices legal clarity. It leaves unresolved whether defensive force rests on independent self-defence claims or on collective security mandates.

7. Institutional Limits and Normative Consequences

The Security Council’s uneven engagement with self-defence against non-state actors has reinforced a decentralized legal environment. States increasingly articulate their own standards, invoking necessity-based reasoning in the absence of clear collective direction.

This trend carries risks. Without stronger institutional guidance, the boundary between lawful self-defence and unlawful use of force becomes blurred. At the same time, the Council’s cautious practice reflects the difficulty of achieving consensus on evolving threats.

8. Implications for the Jus ad Bellum

Security Council practice neither fully resolves nor negates the legal debate over self-defence against non-state actors. Instead, it shapes the context in which the debate unfolds. The Council’s recognition of non-state threats has expanded the conceptual space for self-defence, while its reluctance to articulate firm rules has left critical questions unanswered.

The result is a system in which collective security and unilateral self-defence coexist uneasily. Understanding this dynamic is essential to assessing how the law can adapt without undermining its foundational restraint on the use of force.

X. Normative Fragmentation and Competing Interpretive Models

The legal debates surrounding self-defence against non-state actors have produced a fragmented normative landscape. Rather than converging on a single doctrinal solution, international law currently accommodates multiple interpretive models that coexist in tension. This fragmentation is not accidental; it reflects deeper structural disagreements about the role of sovereignty, the authority of judicial interpretation, and the weight of evolving State practice.

1. Sources of Fragmentation in the Jus ad Bellum

Normative fragmentation arises from the interaction of three factors. First, the UN Charter provides a stable textual framework that has remained unchanged despite profound shifts in the nature of security threats. Second, judicial interpretations, particularly by the International Court of Justice, have tended to emphasize continuity and restraint, often privileging inter-State paradigms. Third, State practice has increasingly asserted flexible approaches to self-defence in response to transnational violence.

These elements pull doctrine in different directions. The Charter anchors legality to a state-centric order, courts reinforce conservative interpretations, and States articulate pragmatic claims driven by perceived necessity. The absence of authoritative reconciliation among these elements generates doctrinal pluralism.

2. The Restrictive Interpretive Model

The restrictive model treats self-defence as tightly circumscribed and fundamentally inter-State in character. Under this approach, armed attacks are understood primarily as acts attributable to States, and defensive force on another State’s territory is lawful only when one of three conditions is met: attribution to the territorial State, valid consent, or explicit collective authorization.

Proponents of this model emphasize legal certainty and systemic stability. They argue that loosening attribution requirements risks hollowing out the prohibition on the use of force and enabling opportunistic interventions. The restrictive model prioritizes institutional authority and seeks to preserve the primacy of collective security mechanisms.

Its principal weakness lies in its limited responsiveness to autonomous non-state violence. Where attribution cannot be established and consent is absent, the model offers little guidance for lawful response, potentially rendering the Charter framework normatively rigid.

3. The Permissive Interpretive Model

The permissive model reorients the analysis toward the factual gravity of the threat. It accepts that non-state actors can generate armed attacks and that self-defence may be exercised against them even in the absence of State attribution, provided strict conditions are satisfied.

This model relies heavily on necessity, proportionality, and temporal constraints as substitutes for attribution. It treats territorial sovereignty as an important but conditional interest, particularly where the territorial State cannot or will not address the threat. The permissive model seeks to align legal doctrine with operational realities and evolving security challenges.

Its vulnerability lies in indeterminacy. Without clear thresholds and external oversight, permissive reasoning risks expanding the scope of unilateral force and weakening the constraint function of the jus ad bellum.

4. Hybrid and Contextual Approaches

Between these poles, hybrid approaches attempt to integrate elements of both models. These approaches accept the possibility of non-state armed attacks while maintaining a strong preference for consent, cooperation, and institutional engagement.

Hybrid reasoning often treats attribution as reinforcing rather than determinative. State involvement strengthens the case for self-defence but is not strictly required. Similarly, the absence of consent triggers heightened scrutiny rather than automatic illegality. These approaches emphasize contextual assessment over categorical rules.

While attractive in theory, hybrid models depend heavily on good faith interpretation and shared understandings that are not always present in practice.

5. Role of Judicial Authority and Its Limits

Judicial interpretation plays a central role in shaping doctrinal coherence. However, courts operate within constraints. They decide specific disputes, often retrospectively, and may be reluctant to articulate broad principles in politically sensitive areas.

This caution contributes to fragmentation. Judicial restraint preserves stability but may lag behind evolving practice. Conversely, reliance on State assertions risks normalizing contested interpretations without sufficient legal consolidation.

The result is a dynamic in which no single actor definitively settles the law.

6. Consequences of Fragmentation

Normative fragmentation has practical consequences. It complicates legal assessment of self-defence claims, weakens predictability, and increases the risk of selective invocation of doctrine. States may choose the interpretive model that best supports their position, reinforcing asymmetry and contestation.

At the same time, fragmentation reflects the law’s struggle to adapt. It signals unresolved tension rather than doctrinal collapse. Competing models coexist because none has achieved sufficient normative authority to displace the others.

7. Toward Conditional Convergence

Despite fragmentation, points of conditional convergence are visible. Across models, there is broad agreement on the importance of necessity and proportionality. There is also shared recognition that self-defence must remain exceptional and that territorial sovereignty cannot be disregarded lightly.

These overlapping commitments suggest that coherence may emerge not through formal codification, but through disciplined application of limiting principles across diverse contexts.

The final section synthesizes these observations and assesses whether the evolution of self-defence against non-state actors reflects adaptive development within the Charter system or signals deeper instability in the jus ad bellum.

XI. Toward a Coherent Legal Framework

The preceding analysis demonstrates that the law governing self-defence against non-state actors has evolved through practice and interpretation rather than formal amendment. The challenge is no longer identifying the existence of competing approaches, but determining how legal coherence can be restored without undermining the Charter’s central restraint on the use of force. A coherent framework must reconcile operational necessity with normative discipline, offering clarity without reverting to rigidity.

1. Re-centering the Armed Attack Threshold

Any coherent framework must begin by reaffirming the armed attack threshold as a meaningful gatekeeper. Recognition that non-state actors can generate armed attacks does not imply that all forms of transnational violence qualify. Only attacks that meet a high threshold of scale, intensity, and consequences should trigger self-defence.

This requirement performs a dual function. It prevents dilution of Article 51 by excluding lower-level violence, and it maintains continuity with the Charter’s original purpose: limiting self-defence to exceptional situations of grave harm.

2. Separating Attribution from Permission

A central source of doctrinal confusion has been the conflation of attribution with the permissibility of defensive force. A coherent framework must distinguish clearly between these inquiries.

Attribution remains essential for determining State responsibility and potential inter-State claims. It should not, however, be treated as a universal prerequisite for self-defence. Where a non-state actor independently generates an armed attack, the absence of attribution does not negate the victim State’s right to defend itself. Instead, it shifts the legal focus to territorial sovereignty, necessity, and proportionality.

This separation preserves the integrity of both legal regimes while preventing attribution from becoming an artificial barrier to lawful defense.

3. Reframing Sovereignty Through Functional Responsibility

Territorial sovereignty must remain a cornerstone of the jus ad bellum, but its application cannot be purely formal. A coherent framework recognizes sovereignty as encompassing both rights and responsibilities.

Where a territorial State exercises effective control and actively suppresses threats, its sovereignty claim is strong. Where it lacks capacity or tolerates armed groups operating against other States, its claim to absolute territorial protection weakens. This reframing does not eliminate sovereignty; it conditions its legal effect on the performance of basic security functions.

4. Constraining the “Unwilling or Unable” Logic

The “unwilling or unable” concept can play a limited role within a coherent framework, but only as a subsidiary necessity test rather than an independent justification.

Its application must be narrow and disciplined. Claims of unwillingness or inability should be supported by objective indicators, such as persistent failure to act, loss of territorial control, or explicit refusal to cooperate. Invocation of the standard should trigger heightened scrutiny, not relaxed legality.

In this framework, the concept operates as an evidentiary mechanism that informs necessity, rather than as a freestanding rule.

5. Elevating Necessity and Proportionality as Primary Constraints

Necessity and proportionality must function as the central legal constraints in self-defence against non-state actors. They should be applied rigorously and transparently.

Necessity requires demonstration that defensive force is the only reasonable means of addressing the threat. Proportionality requires continuous assessment of scope, duration, and intensity. Together, these principles ensure that self-defence remains reactive and limited.

A coherent framework treats violations of necessity or proportionality as fatal to legality, regardless of the identity of the attacker or the invocation of auxiliary doctrines.

6. Strengthening Procedural Discipline and Transparency

Procedural obligations, particularly reporting to the Security Council, play an underutilized role in maintaining coherence. Timely and detailed reporting promotes accountability and enables international scrutiny.

A disciplined framework encourages States to articulate their legal reasoning clearly, including threat assessments, necessity analysis, and the measures taken to limit harm. While procedural compliance does not validate substantive legality, it strengthens the credibility of self-defence claims and facilitates normative convergence.

7. Preserving the Collective Security Orientation

Finally, coherence requires reaffirming that unilateral self-defence operates within, not outside, the collective security system. Defensive force must remain provisional and subordinate to collective responses.

Where possible, engagement with the Security Council and regional mechanisms reinforces legitimacy and reduces the risk of unilateral excess. A coherent framework treats collective security not as an obstacle, but as the ultimate stabilizing reference point.

8. Framework Summary

A coherent legal framework for self-defence against non-state actors rests on the following pillars:

• A strict armed attack threshold grounded in scale and effects.

• Clear separation between attribution and permissibility.

• Sovereignty is understood as both a right and a responsibility.

• Narrow and evidentiary use of the unwilling or unable logic.

• Rigorous application of necessity and proportionality.

• Procedural transparency and reporting discipline.

• Continued primacy of collective security mechanisms.

This framework does not resolve all controversies, but it offers a structured path toward consistency. It preserves the Charter’s core constraints while allowing the law to respond to contemporary threats in a disciplined and legally defensible manner.

XII. Conclusion: Evolution Without Consensus

The development of Self-Defence Against Non-State Actors reflects an adaptive but unsettled evolution of the jus ad bellum. International law has not remained static in the face of transnational armed violence, yet it has not converged on a single, authoritative framework. Instead, doctrine has expanded through practice, interpretation, and necessity-driven argumentation, producing a landscape defined more by managed ambiguity than by consensus.

The core tension remains unresolved. The UN Charter was constructed around an inter-State paradigm that presupposed identifiable attackers, territorial control, and centralized responsibility. Non-state actors capable of sustained, cross-border armed violence destabilize those assumptions. Legal responses have therefore emerged incrementally, often reactively, without formal recalibration of Charter text or institutional mandates.

Judicial interpretation has generally favored restraint, reinforcing attribution-based logic and emphasizing sovereignty as a structural constraint. State practice, by contrast, has increasingly emphasized functional necessity, arguing that the gravity and persistence of non-state attacks can trigger self-defence even absent State attribution. Neither approach has achieved normative dominance. Each addresses genuine concerns while leaving critical gaps.

This divergence explains the current condition of evolution without consensus. The law has shifted enough to recognize that non-state actors can generate armed attacks in a legal sense, yet it has not settled how far sovereignty may be overridden, how necessity should be operationalized, or how unilateral action should be disciplined when collective mechanisms stall.

The risk is not doctrinal disagreement per se, but erosion at the margins. If permissive interpretations expand without firm limits, self-defence risks becoming a flexible justification rather than an emergency exception. If restrictive interpretations remain inflexible, the law risks irrelevance in scenarios where threats are real, immediate, and transnational. Stability depends on navigating between these extremes.

The analysis in this article suggests that coherence is more likely to emerge through disciplined application of limiting principles than through categorical rules. Necessity, proportionality, temporal proximity, and procedural transparency offer common ground across competing models. When applied rigorously, these principles preserve the Charter’s constraint function while allowing measured adaptation.

Ultimately, Self-Defence Against Non-State Actors illustrates a broader pattern in international law: normative evolution driven by practice, constrained by structure, and mediated by institutions that struggle to keep pace with change. The absence of consensus does not signal legal collapse, but it does impose responsibility. States invoking self-defence bear a heightened burden of justification, and institutions tasked with maintaining peace bear a continuing obligation to clarify, scrutinize, and, where possible, consolidate the law.

Until that consolidation occurs, self-defence against non-state actors will remain a doctrine in transition—legally operative, politically contested, and normatively incomplete.

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