Self Defence Against Non-state Actors

International Law’s dialogue with reality: How and to what extent did the events of September 11th 2001 reshape the traditional paradigm of self defence under article 51 of the UN Charter

Introduction

The UN Charter (almost) begins at Article 2 with the premise that ‘All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations’ (art. 2, paragraph 4). This statement naturally recalls the creation of the UN itself, born in the aftermath of the Second World War with the goal of preserving peace and no longer developing international relations by means of war. The prohibition of the use of force, beyond its definition within the Charter, has become integrated in international law as jus cogens (applicable to all states), meaning that any treaty signed between two states authorising the use of force against a third state is void.

This being said, the principle is called back into question in article 51 of the same Charter which contemplates the possibility of legitimate defence in the case of an ‘armed attack’, though it does not specify whether the attack must be made by a state or not. This represents a rather problematic issue considering that the traditional view of international law (i.e. statist or classical) rooted in the Peace of Westfalia (1648), is that only states can be subjects of International Law. How can this classical proposition be then reconciled with transnational or non-affiliated armed groups, such as ISIS and Al-Qaeda?

This essay argues that while post-9/11 state practice has expanded the scope of Article 51 to include self-defence against non-state actors, this development remains doctrinally unstable due to tensions with sovereignty and the ICJ’s jurisprudence.

Self-defence under Article 51 of the UN Charter and its Traditional Interpretation

Article 51 of the UN Charter states the following:

‘Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security’.

The article describes a droit naturel of self-defence in the case of situations which are so sudden that the UN Security Council may not adopt any preventative action. Self-defence, in fact, excludes the illicit nature of a certain response, though it must respond to a) an armed attack, b) necessity, c) proportionality and d) immediateness. It appears evident that self-defence represents an exception with regards to art. 2 paragraph 4.

An interesting development which should be noted under article 51 of the UN Charter is that which was described in the judgment of ‘Military and Paramilitary actions in Nicaragua and Against Nicaragua’ in which the International Court of Justice confirmed that legitimate defence extends beyond article 51 of the UN Charter and is regulated also by ‘general international law’ on the grounds that ‘there is a “natural” or “inherent” right of self-defence and it is hard to see how this can be other than of a customary nature […]’. Furthermore, the same case has been pivotal in the Court’s then newfound need to distinguish between the ‘use of force’ and an ‘armed attack’, where the armed attack represents the most severe form of use of force. However, another interpretive doubt arises as to the notion of ‘armed attack’ where the main question concerns whether or not the so-called preventive legitimate defence, e.g. when there is a threat of armed attack which has not yet occurred, is in fact legitimate.

Moreover, the traditional notion of ‘armed attack’ as adopted in the Nicaragua case of 1986 as well as in the case of Armed Activities on the Territory of the Congo (2005), is that an ‘armed attack’ requires state involvement, and so, in the case of an armed group there must be proof of the group being sent on behalf of a state, has been put into question in the recent years, following the September 11 events which have significantly lowered the ‘effective control’ threshold of attribution, which is no longer required. A lower level of involvement now seems sufficient and the fact that such geopolitical events are relatively recent does not necessarily bar the possibility of a custom formation.

Self-defence in the Post 9/11

In the aftermath of the attacks of September 11, 2001, the question of self-defence against non-state actors became all the more pressing. The United Nations security council immediately condemned the attacks in resolutions 1368 and 1373, recognizing the inherent right of self-defence without mentioning the explicit requirement of state attribution. Further, the United States invoked Article 51 of the UN Charter against Al-Qaeda and the Taliban regime in Afghanistan, treating the events of 9/11 as an armed attack warranting self-defence. The U.S’s approach and its intervention were met with little objection and widespread international support, marking a significant paradigm shift in the interpretation of the legislation. This drastic reappraisal of article 51 of the charter is where most scholars draw a divide between self-defence in the pre and post 9/11 legal climates.

This more lenient interpretation paved the way for other looser interpretive doctrines to be used when applying article 51 of the UN Charter, such as the unable or unwilling doctrine, which grants a state the use of force against a non-state actor when the host state is ‘unable or unwilling’ to contain the threat. This is the rationale that the U.S. invoked to target Al-Qaeda militants sheltering abroad. For example, the 2011 Abbottabad raid that killed Osama Bin Laden was conducted without Pakistan’s consent as the US argued that the nation was unable or unwilling to act against the terrorists.

Nonetheless, this more expansive interpretation of Article 51 to cover non-state actors remains debated. Some scholars argue it represents a necessary adaptation of the article to modern threats, like those mentioned above. On the other hand, numerous states and scholars warn that broadening self-defence in this way risks eroding fundamental UN Charter limits. Brazil and Mexico have explicitly voiced rejection of the ‘unable or unwilling’ doctrine, in fear of undermining the legal order established post WWII. The International Court of Justice has likewise supported a more restrictive view, suggesting that attacks by non-state actors do not trigger Article 51 unless they are attributable to a state, a stance some now deem ‘out of step with the times’ given today’s reality of transnational terrorism.

The post-9/11 practice has undeniably expanded the scope of self-defence to cover non-state actors, but clear consensus on the matter is elusive. The war on terror era prompted a profound debate between those who see a necessary evolution of international law and those who caution that taking a lenient approach to Article 51 threatens to unravel the Charter’s core principles.

Central legal questions in the post 9/11 world

The new readings of article 51 UN Charter in the aftermath of the events of 2001 described above have posited a series of emerging legal doubts and elicited further interpretive queries and questions. Can the definition of ‘armed attack’ be the same for state and non- state actors? Is it possible to reconcile the notion of attribution to the state with independent armed groups? What happens with territorial sovereignty of the state?

As stated previously, not every use of force amounts to an armed attack, which only occurs when force is used the most severe way. To assess severity, the ICJ in the Nicaragua case developed the so-called scale and effects test, through which it has been stated that minor ‘frontier incidents’ or sporadic support to rebels do not amount to an armed attack, only attacks of a wide ranger do. Large-scale terrorist incidents can amount to an armed attack, whereas isolated events of the sort may not. The measure of ‘severity’ however still remains unclear.

Another thorny issue is linked to state attribution, as the ICJ has consistently stressed that violence by private groups may only trigger Article 51 if attributable to a state. However, in 2001 the UN Security Council recognized the inherent right to self-defence in response to the purely non-state related 9/11 attacks. Nonetheless, the ICJ and some jurists remain cautious, so the necessity of state attribution post-9/11 remains an unsettled question in international law.

A further legal tension concerns sovereignty when a state uses force against non-state actors on another state’s territory without consent, a phenomenon that runs along the lines of the unable or unwilling doctrine. Such cross-border incursions on territory evidently violate state sovereignty (UN Charter Article 2(4)), even if purported as non-statewide threats. Proponents of a stricter view of this article emphasize that the victim state’s right of self-defence must be balanced against the target state’s territorial integrity. By invoking the unable or unwilling doctrine, respect for sovereignty is maintained by treating cross-border force as a last resort, necessary only when the territorial state fails to neutralize the non-state actor. Nevertheless, this ‘last resort’ remains highly contested. In fact, scholars note that only some states, like the U.S that has historically adopted an interventionist policy, have formally invoked ‘unwilling or unable’ in legal justifications, and that an abuse of the unable or unwilling test may result in an erosion of sovereignty and set a dangerous precedent for abuse. Determining when a state is truly ‘unable’ or ‘unwilling’ is also problematic. Thus, while the doctrine has been invoked in self-defence claims to justify operations against terrorists in weak states, its status in international law is uncertain and remains rather divisive.

Finally, the issue of collective self-defence arises in the context of non-state actors. Collective self-defence is permitted by article 51 of the UN charter and is such when one state is attacked (even by a non-state group), and other states may assist it at the request of the victim state. This idea was invoked after 9/11, when NATO allies treated the attacks on the US as triggering collective defence obligations, even if those attacks were perpetrated by a non-state entity. Similarly, in recent years, states have combined collective self-defence with the unwilling/unable logic. For example, when Iraq’s government faced attacks by the ISIS group, it invited coalition partners to help fight ISIS. Those partners reported to the UN that they were acting in collective self-defence of Iraq by striking ISIS bases across the border in Syria. They emphasized that their operations were directed only at the non-state actor and not against Syria’s regime, aiming to stay within Article 51’s bounds in order to respect Syria’s sovereignty. This example underscores the dual nature of collective self-defence against non-state actors: it can provide legal justification for multinational responses to terrorism, but it also blurs the line between consensual assistance and undesired intervention, especially when the territorial state’s consent is absent. Each of these issues, such as armed-attack thresholds, attribution, sovereignty conflicts, the unable/unwilling test, and collective self-defence, reflect the evolving and disputed nature of self-defence against non-state actors under Article 51 in the post 9/11 world.

9/11’s Impact on the Self-defence Paradigm

As discussed above, Article 51 of the UN Charter’s paradigm was largely state centric prior to 2001. With the aforementioned concern for the respect of territorial sovereignty and the mitigation of the issue with the notion of state attribution, terrorist attacks perpetrated by private groups therefore represented a grey area: they were crimes and potential acts of war, but not clearly ‘armed attacks’ under Article 51 unless a link to a state could be shown. The ICJ echoed the narrow interpretation of UN Charter Art 51, on the grounds that the provision of the article does not mention whether the attacker must be a state. Prevailing pre-9/11 legal opinion treated Article 51 self-defence as an inter-state right, in cases like the 2004 Wall Advisory Opinion.

The September 11 2001 attacks, however, signalled a fundamental paradigm shift. NATO invoked Article 5 of its treaty for the first time in solidarity with the U.S. Virtually no state criticized the U.S. military response against Al Qaeda and the Taliban in Afghanistan; on the contrary, there was broad support. Observers noted the ‘relative absence of criticism from the international community’ – the 9/11 atrocity was widely seen as an act of war, and the U.S.-led military response ‘required no explanation or justification’. For the first time, self-defence was openly invoked against a non-state actor on another state’s territory. This interpretive adjustment also came along with a significant reshapement of the doctrinal framework. Firstly, the scale and impact of 9/11 led to an understanding that a particularly devastating terrorist attack may amount to an ‘armed attack’ in the Charter sense. This consensus broke the earlier taboo and set a precedent, granting states leeway to interpret Article 51 more broadly.

Secondly, the post-9/11 era developed the ‘unwilling or unable’ doctrine into state practice. This concept emerged because 9/11 demonstrated the danger of safe havens: Al Qaeda found shelter by the Taliban in Afghanistan, who were unwilling to hand them over. The U.S. and its allies thus asserted that they were entitled to act in Afghanistan because the host state failed to stop the attackers. Over time, this reasoning solidified. States like Turkey, Colombia, Israel, and the U.S. have echoed the unwilling or unable rationale. While still debated in scholarship, such practice indicates the affirmation of a permissive trend: a state’s sovereignty may be suppressed or even entirely ignored if the state in question fosters (actively or passively) a serious terrorist threat. This is a significant departure from the pre-9/11 paradigm, which placed greater emphasis on territorial sovereignty.

The temporal factor is another factor that has been reshaped by the events of 9/11. Classical doctrine held that self-defence should be a response to an ongoing or imminent armed attack. However, 9/11 showed that terrorist threats may be intermittent, blurring the line between past attack and future risk. In fact, the U.S. response occurred almost a month after September 11 2001, and was justified on the grounds of preventing future terroristic attacks. Terrorism in fact represents a type of attack that can be difficult to reconcile with the traditional requirement of immediacy.

Hence, the events of 9/11 expanded both the entities against whom self-defence can be invoked and, to some extent, the circumstances under which it can be exercised.

A current example: The Israel-Hamas conflict

Hamas is a non-state armed group operating from Gaza and, after the October 7, 2023, cross-border assault, Israel has consistently invoked its inherent right of self-defence under Article 51 of the UN Charter, supported by other states like the U.S was two decades ago. If we apply the logic of 9/11, Hamas’s coordinated assault clearly qualifies as an armed attack, and Israel may respond forcefully against the perpetrator (Hamas).

However, the Israel–Hamas situation also highlights the legal complexities that still exist, even in the post-9/11 paradigm. For example, Gaza’s legal status is contested, and many countries (and the International Court of Justice) consider Gaza, along with the West Bank, as territory under Israeli occupation, which naturally implies that Article 51 self-defence might not apply in the same way.

The Israel–Hamas conflict exemplifies the concrete application of the post-9/11 self-defence paradigm and where it remains contentious. It demonstrates that the international legal system, by and large, has accepted that non-state actors can unleash ‘armed attacks’ and face military responses. Yet it also shows that circumstances matter. Notably, even those who agree Israel had a right to respond to Hamas caution that the manner of response must obey international humanitarian law within the limits of necessity and proportionality under jus ad bellum and may not grant a license for revenge or collective punishment.

Conclusion

In its dialogue with reality, international law has bent to adjust to reality: states now generally accept that self-defence can be invoked against significant armed attacks by non-state actors. The events of September 11th profoundly reshaped the Article 51 paradigm, adjusting to a model which is able to respond to modern threats. Nonetheless, these changes do not give states a blank check to wage unlimited war under the pretext of counterterrorism and they must still comply with requirements of necessity and proportionality. Nonetheless, in practical and political terms, 9/11’s legacy represents the cause of the expansion of scope for Article 51. International law has updated the notion of self-defence to bring the law up to date with recent historical developments, but the verdict on such changes remain unclear as scholarship and the ICJ do not appear entirely convinced.

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