Scope, power, and competing theories in contemporary international law
The right of states to use force in self-defence occupies a central yet deeply contested place in contemporary international law. Enshrined in Article 51 of the United Nations Charter as an exception to the general prohibition on the use of force in Article 2(4), self-defence has traditionally been understood as a narrowly circumscribed right exercised in response to an armed attack attributable to another state. However, the rise of transnational armed groups and the security shocks of the post-9/11 era have placed unprecedented strain on this framework. This blog examines the scope of self-defence against non-state actors and situates the debate within broader theoretical disagreements between realism and institutionalism in international security studies.
From inter-state self-defence to non-state threats
Classical international law conceived self-defence as a right operating primarily between states. Armed attacks triggering Article 51 were expected to be imputable to a state, and the legality of any defensive response depended on strict compliance with customary requirements of necessity, proportionality, and immediacy, famously articulated in the Caroline correspondence. Respect for territorial sovereignty further required that force on another state’s territory be lawful only with consent or Security Council authorisation.
A central difficulty in extending self-defence to non-state actors lies in attribution. The International Court of Justice has consistently favoured a restrictive “effective control” test, first articulated in Military and Paramilitary Activities in and against Nicaragua (ICJ, 1986) and reaffirmed in subsequent cases such as Oil Platforms (ICJ, 2003). Under this standard, a state is responsible for the acts of a non-state group only if it directed or enforced the specific operation in question.
By contrast, the International Criminal Tribunal for the former Yugoslavia adopted a more flexible “overall control” test in Prosecutor v Tadić (ICTY Appeals Chamber, 1999), allowing attribution where a state organises, coordinates, or supports a group’s general activities. Despite this divergence, the ICJ has repeatedly reaffirmed the stricter threshold, most notably in Armed Activities on the Territory of the Congo (ICJ, 2005), where Uganda’s claim of self-defence against armed groups operating from the DRC was rejected due to insufficient proof of state attribution.
This jurisprudence leaves international law relatively clear on one point. Where non-state actors operate autonomously, without the requisite degree of state control, the traditional law of self-defence offers no straightforward answer. It is precisely this gap that became impossible to ignore after the attacks of 11 September 2001.
9/11 and the “unwilling or unable” turn
The 9/11 attacks marked a pivotal moment in the legal and political understanding of self-defence. For the first time, the use of force in response to attacks by a non-state actor was widely treated as potentially lawful even without clear attribution to a territorial state. The Security Council resolutions adopted in the immediate aftermath, notably Resolution 1368 (2001) and Resolution 1373 (2001), affirmed the inherent right of self-defence in response to the attacks, implicitly accepting that non-state actors could, under certain conditions, launch armed attacks within the meaning of Article 51.
This shift has been accompanied by the emergence of the so-called “unwilling or unable” doctrine. According to this approach, a state may use force in self-defence on the territory of another state if that state is unwilling or unable to suppress the threat posed by non-state actors operating there. The doctrine has been analysed extensively by scholars such as Ashley Deeks (2012) and Daniel Bethlehem (2012). While several Western states have relied on this reasoning to justify operations in Syria and Pakistan, many states from the Global South continue to reject its status as customary international law, warning that it risks eroding the prohibition on the use of force.
A TWAIL critique of post-9/11 “newness”
From a Third World Approaches to International Law perspective, the post-9/11 expansion of self-defence is not merely a technical legal development but a deeply political project. Obiora Okafor’s critique of the “newness claim” highlights how powerful states portray certain events as so unprecedented that they allegedly justify retrenching long-standing legal constraints (Okafor 2005).
From this viewpoint, framing 9/11 as a singular rupture in international history obscures the lived experiences of many societies in the Global South that have long endured mass violence and foreign intervention. Drawing on the work of Richard Falk, Okafor challenges the narrative that “megaterrorism” is uniquely transformative, pointing to historical episodes such as US bombings in Japan, Angola, El Salvador, and Mozambique as evidence that large-scale violence is not a post-9/11 novelty. The concern, from a TWAIL perspective, is that legal reform driven by such narratives risks reproducing older imperial patterns in which international law adapts primarily to the security anxieties of powerful states.
Realism and the logic of survival
These legal developments resonate strongly with realist theory in international relations. John J. Mearsheimer’s structural realism argues that the anarchic nature of the international system compels states to prioritise survival above all else (Mearsheimer 1994/95). In a world without a central authority, uncertainty about others’ intentions and the catastrophic costs of inaction incentivise unilateral action, even where legal constraints are stretched.
From this perspective, the post-9/11 expansion of self-defence against non-state actors is not an aberration but a predictable response to systemic pressures. When institutions appear unable to provide timely or reliable protection, powerful states are likely to reinterpret or contest legal norms in ways that favour pre-emptive or preventive action. International law, on this view, does not disappear, but its application is shaped by power and security imperatives rather than normative consensus.
Institutionalism and the case for legal restraint
Institutionalist theory offers a more optimistic counterpoint. Robert Keohane and Lisa Martin argue that institutions can meaningfully influence state behaviour by reducing uncertainty, facilitating cooperation, and structuring expectations of reciprocity (Keohane and Martin 1995). Institutions matter not because they eliminate self-interest, but because they alter the strategic environment in which self-interest is pursued.
Applied to self-defence, institutionalism cautions against unilateral expansions such as the “unwilling or unable” doctrine. Allowing states to determine unilaterally when force is lawful risks normalising the use of force and undermining collective security mechanisms. From this perspective, responses to non-state threats should be embedded within multilateral frameworks, including clearer legal standards, intelligence-sharing, and meaningful Security Council oversight. While imperfect, such institutional constraints reduce mistrust and limit escalation in ways unilateral action cannot.
Conclusion
The debate over self-defence against non-state actors lies at the intersection of law, power, and theory. The post-9/11 era has exposed both the limitations of a legal framework built around inter-state conflict and the dangers of loosening foundational constraints on the use of force. Realism explains why states are tempted to stretch legal norms in the face of uncertainty and fear. Institutionalist and TWAIL perspectives remind us why such moves are normatively troubling and structurally risky, particularly for weaker and historically marginalised states.
Whether international law will evolve toward a broadly accepted framework governing self-defence against non-state actors remains uncertain. What is clear is that the outcome will depend not only on doctrinal refinement, but on deeper struggles over power, legitimacy, and whose security concerns international law ultimately serves.
