The Unwilling or Unable Doctrine in International Law

1. The legal question

The unwilling or unable doctrine is invoked to argue that a State that has suffered an armed attack by a non-State armed group may use force against that group in the territory of another State, even without the territorial State’s consent, if the latter is unwilling or unable to suppress the threat. The two conditions are alternatives, not cumulative requirements. An example is an organised armed group that launches attacks across a border from territory no longer controlled by the State in which it is located.

The legal difficulty lies at the intersection of the prohibition on the use of force in Article 2(4) of the UN Charter, the right of self-defence in Article 51, and the territorial State’s sovereignty. The mere fact that an attack originates from State B does not establish that the attack is legally attributable to State B.

2. Two distinct legal questions

First, can a sufficiently grave attack conducted by a non-State actor amount to an armed attack triggering Article 51 even when the act is not attributable to a State? Second, assuming the answer is yes, may defensive force be exercised in the territory of a non-consenting State that is not responsible for the attack? The unwilling or unable doctrine primarily addresses the second question. Invoking the doctrine does not itself resolve the first.

Not every terrorist act or security risk reaches the threshold of an armed attack. Nor does the doctrine, standing alone, establish a right to preventive attacks against speculative or remote threats. Claims of self-defence against imminent attacks raise further contested questions.

3. The two limbs of the proposed test

Unwilling refers, in the formulations advanced by the doctrine’s proponents, to a territorial State’s refusal or failure to take effective measures against the armed group despite being in a position to do so. An unwillingness finding cannot simply be inferred from a disagreement over counterterrorism policy or an unsubstantiated allegation of non-cooperation. Nor does it, without more, establish the territorial State’s direction, control, complicity or responsibility for the group’s attack.

Unable refers to a territorial State’s lack of practical capacity to take effective measures against the armed group, even if it is willing to act. A typical illustration is the loss of effective control over the area from which the group operates. Inability does not automatically amount to a breach of international law: the obligation to exercise due diligence should not be confused with a guarantee that all hostile acts originating from a State’s territory will be prevented. Nor does incapacity diminish that State’s sovereign rights.

More restrictive formulations call for an evidence-based assessment of the gravity of the attacks, actual territorial control, the State’s capabilities and actions, opportunities for cooperation, any request for assistance, and the availability of less intrusive measures. There is, however, no universally accepted procedure or evidentiary standard for applying the test.

4. Necessity, not an independent exception

The US Government has described the unwilling or unable standard as an application of the necessity requirement for self-defence. If the territorial State can effectively address the threat, a cross-border operation would not be necessary; if it genuinely cannot or will not do so, direct defensive force might be necessary. On this view, the doctrine seeks to constrain rather than abolish respect for territorial sovereignty.

The argument does not dispense with the other conditions of self-defence. An armed attack of sufficient gravity must be established. Force must be necessary and proportionate to the defensive purpose, including in its scope, duration and intensity. Article 51 separately requires that measures taken in the exercise of self-defence be immediately reported to the Security Council. Military action cannot be transformed into lawful defence merely by calling a punitive operation or the occupation of another State’s territory a response to terrorism.

The central objection remains: why does the necessity of countering a non-State actor provide legal authority to use force in a non-consenting third State? Necessity may limit a legal entitlement, but whether it can also justify encroachment upon a third State’s sovereignty is precisely what is disputed.

5. Syria and ISIS: the principal precedent

The most frequently discussed application occurred in September 2014. The United States launched strikes against ISIS/ISIL (Daesh) in Syria and, in its letter of 23 September 2014 to the UN Secretary-General (S/2014/695), invoked Article 51, including the collective self-defence of Iraq, and argued that the Syrian Government was unwilling or unable to prevent the use of its territory to mount attacks.

The territorial distinction is crucial. The Iraqi Government consented to military action on Iraqi territory. The US-led operations in Syria did not have the consent of the Syrian Government. Iraq’s request for collective self-defence did not, by itself, answer the separate question of the legality of force on Syrian territory.

Other participating States relied on comparable but not always identical legal theories. Germany’s letter of 10 December 2015 (S/2015/946) emphasised the Syrian Government’s lack of effective control over the parts of Syria occupied by ISIL, reasoning that States attacked from those areas could exercise self-defence even without Syrian consent. By contrast, the United Kingdom’s letter of 7 September 2015 (S/2015/688) invoked individual and collective self-defence without itself explicitly setting out the unwilling or unable formula. It would be inaccurate to attribute one uniform legal justification to every member of the coalition.

Security Council Resolution 2249 (2015) called upon States to take necessary measures against ISIS in accordance with international law. It did not clearly confer a stand-alone Chapter VII authorisation to use force and did not establish the unwilling or unable doctrine as a binding rule.

6. The International Court of Justice

In Nicaragua v. United States (1986), the ICJ considered when the dispatch of armed bands by or on behalf of a State, undertaking operations of sufficient scale and effects, could constitute an armed attack. This case is central to the role of State involvement in the traditional framework; it does not exhaust the present debate about autonomous non-State groups.

In the Wall advisory opinion (2004), paragraph 139, the Court referred to Article 51 in the context of armed attack by one State against another and stressed that the threats invoked by Israel originated in territory under its own control. The reasoning must be read in context. Certain separate opinions criticised an overly restrictive reading of Article 51.

In Armed Activities on the Territory of the Congo (DRC v. Uganda) (2005), paragraphs 146–147, the Court found no satisfactory proof that the attacks invoked by Uganda were attributable to the DRC and that the requirements for Uganda’s claim of self-defence were not met. Crucially, the Court expressly declined to decide whether, and under what conditions, large-scale attacks by irregular forces could trigger self-defence. The judgment cannot be cited as an endorsement of the unwilling or unable test; nor does it settle every possible non-State actor scenario.

Security Council Resolutions 1368 and 1373 (2001) referred to the inherent right of self-defence in the aftermath of the 11 September attacks. They did not explicitly establish the unwilling or unable standard.

7. Customary international law and the 2025 Mexican position

Whether the doctrine has become customary law depends on State practice and opinio juris. Supporters maintain that a significant practice of extraterritorial defence and legal justification has developed. Critics point to limited or inconsistent positions, express opposition, and the difficulty of treating silence as acceptance.

A major recent official contribution is A/79/719–S/2025/26, Mexico’s position paper sent to the UN on 10 January 2025and distributed on 13 January. Mexico expressly rejected the doctrine as a permissible basis for invoking Article 51 and warned against unilateral, expansive uses of self-defence. Its position is more restrictive than opposition to the unwilling or unable test alone: paragraph 25 also rejects the proposition that attacks by irregular armed groups, without attribution to a State, qualify as armed attacks for the purposes of Article 51. Mexico thus disputes both legal propositionsdistinguished at the beginning of this article. This document reflects Mexico’s legal position, not a binding decision or an institutional determination of the United Nations merely because it was circulated as an official UN document.

The academic debate is also divided. Lucy V. Jordan argues that the standard has attained customary status. Olivier Corten argues that it has not won sufficiently general acceptance. Nicholas Tsagourias, writing in 2025, likewise regards the test’s status as unsettled. He advances a different account, treating self-defence as a primary rule permitting a defensive response to an armed attack irrespective of its perpetrator and discussing its possible effect as a circumstance precluding wrongfulness in relation to incidental infringements of the territorial State’s sovereignty. This is an academic argument, not an established judicial holding. It is therefore inaccurate to present the test as an uncontested rule of positive international law.

8. Principal objections

Territorial sovereignty: inability to control an armed group is not consent to foreign military operations; neither is it automatically attribution of the group’s conduct to the territorial State.

Unilateral assessment: the State using force frequently decides for itself whether the territorial State is unwilling or unable. Without agreed evidentiary and procedural criteria, the test creates a substantial risk of abuse.

Expansion of Article 51: a generic terrorist threat, or the mere presence of an armed group, must not be confused with an armed attack meeting the requisite threshold.

The limits of necessity: demonstrating that an operation would be useful or even operationally indispensable does not alone explain the legal basis for crossing the borders of a non-consenting State.

9. Distinction from international humanitarian law

The doctrine concerns jus ad bellum, not jus in bello. Even a lawful act of self-defence remains subject to international humanitarian law whenever an armed conflict exists. Targeting, distinction, proportionality of attacks, and precautions must be assessed independently. Conversely, the lawfulness of targeting an object under international humanitarian law does not, by itself, authorise the use of force in another State’s territory. International human rights law may also apply according to its own jurisdictional conditions.

10. Classroom hypothetical

An organised armed group G, based in State B, launches repeated large-scale attacks on State A. B condemns G but no longer controls the region from which it operates. A proposes limited strikes against G’s weapons depots, without B’s consent.

A proponent of the test might argue that B’s inability makes defensive action by A necessary. A critic would ask whether an armed attack by G can justify crossing B’s border when G’s conduct is not attributable to B and B has withheld consent. Both must consider separately the gravity of G’s acts, Article 51, necessity and proportionality, Security Council reporting, and the rules applicable to the specific attack under humanitarian law.

Conclusion

The doctrine addresses a real operational problem: States can face grave armed attacks originating in territory that the territorial sovereign cannot effectively control. The legal response, however, has to account for both the victim State’s security and the territorial State’s rights. The proposed test remains a contested interpretation supported by some States and scholars, but disputed as to its legal basis, content, and customary status.

Selected sources

Primary sources and official positions

Scholarship