The Unwilling and Unable Doctrine and Syria

Written by Jefferi Hamzah Sendut

The ongoing US-led military campaign against terrorist groups1 operating in Iraq and Syria dubbed Operation Inherent Resolve has raised vital questions over the legality of the use of force in self-defence against non-State actors. The right to self-defence asserted in line with an ‘unwilling or unable’ doctrine as the legal justification for Operation Inherent Resolve’s targeting of Islamic State in Iraq and Syria (ISIS)2 and Khorasan Group terrorist positions in Syria is without firm basis in customary international law. This piece suggests that the States militarily involved in Operation Inherent Resolve should therefore make clear that their action in self-defence is exceptional, rather than relying on a doctrine problematic on a policy level, particularly in view of the centrality of the UN Charter Article 2(4) prohibition on the use of force to the international system.

The Syrian Civil War and the unwilling or unable doctrine

Since the start of the conflict in March 2011, the Syrian Civil War has become characterised by external intervention in support of belligerents both allied to, and fighting against the Assad regime.3 Amongst the numerous non-State actors involved are extremist groups internationally regarded as terrorist organisations, including ISIS and Khorasan Group.

Following the ‘rapid territorial growth’4 of ISIS, in particular through the capture of Iraqi and Syrian territory in 2014, the US, together with Australia, Canada and the UK, began an aerial bombing campaign (hereafter ‘Operation Inherent Resolve’)5 targeting terrorist positions in Iraq following the Iraqi government’s invitation.6 More contentiously, Operation Inherent Resolve also initiated strikes against terrorist positions within Syria without the consent of the Assad regime, while accepting it constitutes the Syrian government.7 The US has argued that it is entitled to take these steps on the basis of both the collective self-defence of Iraq against ISIS, as well as in individual self-defence.8 It has advanced an ‘unwilling or unable’ doctrine to support a claim to a right of self-defence covering its conduct. A right to use necessary and proportionate force against terrorist groups is asserted as Syria is ‘unwilling or unable to prevent the use of its territory’ for terrorist attacks.9 For the Syrian government’s part, it maintains that the US-led strikes are unlawful.10 It regards its consent as legally required for anti-terror activities on Syrian territory, and that effective steps to combat terrorism have been taken despite harmful external intervention.11

What does the doctrine entail?

It is useful to outline how the unwilling or unable doctrine, if accepted in law, would alter the traditional12 scope of the ‘inherent’ customary international law right of States to self-defence in response to an ‘armed attack’, codified in Article 51 of the UN Charter. The two main distinguishing features entailed by the doctrine are: (i) a right to self-defence against non-State actors in the absence of State responsibility through ‘effective control’, and (ii) a right to pre-emptive self-defence. For a right to self-defence within the parameters of this doctrine to be available to the Operation Inherent Resolve States, ‘settled practice’ of States accompanied by opinio juris is required.13

Self-defence against non-State actors in the absence of Nicaragua State responsibility

Self-defence according to the unwilling or unable doctrine considers that an armed attack may be committed by a non-State actor. That a non-State actor may physically commit an armed attack is in itself uncontroversial – it is a possibility not precluded by the text of Article 51. Indeed, the ICJ in Nicaragua confirmed that the actions of irregulars can constitute an armed attack where of similar ‘scale and effects’ to an armed attack by regular forces, applying Article 3, paragraph (g) of the Definition of Aggression.14

The key divergence between the position set out in Nicaragua and the unwilling or unable doctrine is the degree of State involvement required for the physical conduct of non-State actors to constitute an armed attack and trigger the right to self-defence. Only self-defence responding to ‘the sending by a State of armed bands to the territory of another State’,15 and therefore an armed attack committed by a State, was in Nicaragua countenanced. The ICJ’s judgement clarified that the law on the use of force and the law on State responsibility cohered in this regard. Earlier in the judgement, it laid out that for the legal responsibility of the US to be engaged, ‘it would in principle have to be proved that that State had effective control of the military or paramilitary operations [in question]’.16 The Court stated that US assistance to the contra rebels, which it also found was not an armed attack,17 did not ‘[warrant] the conclusion that [the contras were] subject to the [US] to such an extent that any acts they…committed [were] imputable to that State’.18 Hence, it is sound to posit that in customary international law at the time of the judgement, at least in the ICJ’s view, there needed to be ‘effective control’ by a State of non-State actors for their conduct to trigger a right to self-defence.19 Although the ICJ’s lack of reference to State practice and opinio juris to support its ‘effective control’ test has been criticised,20 States have demonstrated that attribution or imputation on the test’s grounds was subsequent to Nicaragua, a requirement to exercise the right to self-defence, at least where concerning terrorism.21 Only isolated instances of State practice such as from Senegal, Tajikistan and Thailand asserting a right to self-defence against non-State actors without State attribution before 9/11 are evident deviations.22

In contrast, the unwilling or unable doctrine either entails that the right to self-defence can be triggered without any attribution of non-State actor conduct to a State, or that non-State actors’ conduct can be attributed without effective control of a State. It cannot be said with total certainty which of these alternatives is being asserted in the context of Operation Inherent Resolve. Nevertheless, it appears likely that given the States involved have not claimed a right to use force in self-defence against Syrian government forces, the former right is claimed.

(ii) Pre-emptive self-defence

The States advocating the unwilling or unable doctrine claim a right to pre-emptive self-defence. In her letter to the Security Council setting out the doctrine, US Ambassador Power makes reference to the abstract ‘threat’ posed by terrorist groups, rather than claiming a right to self-defence in response to a specific armed attack. The position has been taken that ‘the nature of [ISIS’] ideology and its modus operandi meant that it poses a continuous threat to the [US], [UK] and others and that, therefore, those States should have a right to respond in self-defence’.23

This is a significant break from the practice of the majority of the international community, which regards even a right to anticipatory self-defence responding to an imminent attack as contentious. Even after 9/11,24 ‘[t]he majority of [States] reject anticipatory self-defence’.25 As explained by Gray, ‘it is only where no conceivable case can be made that there has been an armed attack that [States, namely the US, UK and Israel have] resort[ed] to anticipatory self-defence’.26 For instance, an unusual pre-9/11 express claim to anticipatory self-defence was made by Israel in relation to its airstrike destroying the Iraqi Osirak nuclear reactor, which Israel claimed would be used for the Iraqi production of nuclear weapons. This was however met with forceful international criticism, with 109 States voting in favour of a General Assembly resolution condemning the airstrike as an act of aggression,27 and numerous States declaring their opposition to a right to anticipatory self-defence.28 Also, the terms of UN Charter Article 51 on a natural reading themselves indicate that a right to self-defence arises in response to an armed attack which has already been initiated.

Anticipatory self-defence was a feature of the customary international law predating the UN Charter, expressed in the Caroline exchange of letters.29 Certain commentators maintain that despite the aforementioned contrary State practice and the plain text of Article 51, because the provision preserves the ‘inherent’ customary international law right to self-defence, a right to anticipatory self-defence thereby remains available to States.30 If one were to accept this position, a disjunct between a right to anticipatory self-defence and the truly pre-emptive self-defence claimed in the unwilling or unable doctrine still remains.

An insufficiency of State practice and opinio juris

Having outlined the disparities between pre-9/11 customary international law and the claims of the unwilling or unable doctrine, the impact of State practice and opinio juris demonstrated subsequent to the 9/11 attacks will now be analysed. It is submitted that there is insufficient State practice and opinio juris to support the unwilling or unable doctrine’s status in law.

Reception to States’ use of the unwilling or unable doctrine in respect of Syria

The unwilling or unable doctrine has received explicit support from a number of States. Aside from the US, the other three States which were militarily involved in Operation Inherent Resolve strikes in Syria at its inception have also espoused the doctrine.31 Canada, which suspended its combat participation in favour of a support role against ISIS, did not do so out of any concern over Operation Inherent Resolve’s legal basis – rather, the decision was based on domestic political concerns over the merits of military intervention generally.32 Alongside these four, Turkey has expressed support for the essentials of the doctrine in an Article 51 letter to the Security Council reporting its own use of force in self-defence against ISIS in Syria, albeit using a marginally different formulation describing the Syrian government as ‘neither capable of nor willing’.33 In opposition, some States have joined Syria in explicitly rejecting the doctrine’s status in law, Russia doing so34 along with Cuba,35 Ecuador36 and Venezuela.37

Less clear cut cases are abundant. Corten38 identifies that many States, without unequivocally rejecting the doctrine, sought to reaffirm generally traditional State sovereignty after airstrikes began, in line with an orthodox, wide reading of the UN Charter’s Article 2(4) prohibition on the use of force. For example, China highlighted that it was ‘imperative to consistently comply with the purposes and principles of the Charter of the United Nations as well as the basic norms governing international relations, while maintaining the sovereignty, independence, unity and territorial integrity of Syria’.39 Angola and Kazakhstan also made statements to that effect.40

Others placed emphasis on multilateralism and cooperation to combat terrorism, seemingly implying a contrast with the contentious route taken by Operation Inherent Resolve. These include Belarus, Brazil, Chad, India, South Africa, and the ten States of the Association of South East Asian Nations.41

As such, it is sound to conclude that the unwilling or unable doctrine, at least in respect of its latest application in Syria, has not been accepted by the international community so as to amount to ‘settled practice’ to widen the concept of armed attack and the right to self-defence.


To conclude, the unwilling or unable doctrine espoused by the US and other States militarily involved in Operation Inherent Resolve is not reflective of the scope of the customary international law right to  self-defence. State practice and opinio juris currently demonstrated in connection with the situation in Syria reveals little support for the doctrine beyond those States actually involved in the campaign. This is a conclusion supported by policy considerations – the unwilling or unable doctrine lacks a defined scope, and would allow the use of force on readings of factual circumstances which are difficult to objectively ascertain with any degree of certainty.


  1. A note on terminology: the term ‘non-State actor’ is used throughout this piece to refer to any individuals or groups not formally components of a State’s military apparatus, unless they are entities listed on the UN Security Council’s ISIL (Da’esh) & Al-Qaida Sanctions List regarded as terrorist groups; although consensus exists over the designation of these entities as terrorist groups, difficulties persist as to defining entities as terroristic generally – see the discussion in Sami Zeidan, ‘Desperately Seeking Definition: The International Community’s Quest For Identifying The Spectre of Terrorism’ [2004] Cornell Int’l L.J 491.
  2. Also known as Islamic State in Syria and the Levant (ISIL), Islamic State (IS), and Dae’sh.
  3. See Barry Posen, ‘Civil Wars & the Structure of World Power’ [2017] Daedalus 167.
  4. Combined Joint Task Force – Operation Inherent Resolve, ‘History’ (Operation Inherent Resolve, July 2017) <> accessed 19 December 2017.
  5. Operation Inherent Resolve also includes a wider coalition of 70 other States and institutions providing non-combat support: Global Coalition, ’74 Partners’ (Global Coalition, 2017) <> accessed 19 December 2017.
  6. See Karine Bannelier-Christakis, ‘Military Interventions against ISIL in Iraq, Syria and Libya, and the Legal Basis of Consent’ [2016] Leiden Journal of International Law 743.
  7. Christian Henderson, ‘Editorial Comment: The Use of Force and Islamic State’ [2014] Journal on the Use of Force and International Law 209, 213.
  8. UN doc S/2014/695.
  9. Ibid.
  10. UN docs S/2015/719, S/2015/1048.
  11. Ibid.
  12. Referring to the period between the coming into force of the UN Charter and the 9/11 terrorist attacks; as is explained below however, the international community’s response to 9/11 was in any case not as transformative of customary international law as has been posited by some commentators.
  13. North Sea Continental Shelf, Judgement, ICJ Reports (1969) 3 at para 77.
  14. Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America). Merits, Judgement, ICJ Reports (1986) 14 at para 195.
  15. Ibid.
  16. Ibid. at para 115.
  17. Ibid. at para 195.
  18. Ibid. at para 116.
  19. See Vladyslav Lanovoy, ‘Use of Force by Non-State Actors and the Limits of Attribution of Conduct’ [2017] EJIL 563.
  20. Antonio Cassese, ‘The Nicaragua and Tadić Tests Revisited in Light of the ICJ Judgment on Genocide in Bosnia’, [2007] EJIL 649.
  21. See Antonio Cassese, Terrorism, Politics and the Law: The Achille Lauro Affair (1989), 996.
  22. UN Doc S/26092 (16 July 1993), Keesing’s 106 (1995) 40554.
  23. Christian Henderson, ‘The use of force and Islamic State’ [2014] Journal on the Use of Force and International Law 207, 218.
  24. And certainly before.
  25. Christine Gray, International Law and the Use of Force (3rd edn, OUP 2008), 160.
  26. Ibid. at 161
  27. GA Res 36/27 (1981).
  28. SC 2288th meeting (1981) para 156.
  29. Jennings (1938) 32 AJIL 82.
  30. E.g. Myers McDougal and Florentino Feliciano, Law and Minimum World Public Order (Yale University Press, 1961).
  31. UK: David Cameron, ‘David Cameron’s full statement calling for UK involvement in Syria air strikes’ (The Telegraph, 26 November 2015)  <> accessed 19 December 2017; Australia: UN doc S/2015/693; Canada: UN doc S/2915/221.
  32. Bill Curry and Steven Chase, ‘Canada will pull jets from combat role before end of March: Trudeau’ (The Globe and Mail, 16 November 2015) <> accessed 19 December 2017.
  33. UN doc S/2015/563.
  34. See Russia’s remarks at the Security Council’s 7271st meeting, UN doc S/PV.7271, 19 September 2016
  35. Olivier Corten, ‘The ‘Unwilling or Unable’ Test: Has it Been and Could it be, Accepted?’ [2016] Leiden Journal of International Law 789.
  36. Ecuador Foreign Ministry, ’Ecuador rechaza ofensiva estadounidense en territorio sirio’ (Ecuador Foreign Ministry) <> accessed 19 December 2017.
  37. See Venezuela’s remarks at the Security Council’s 7504th meeting, UN doc S/PV.7504, 17 August 2015.
  38. Olivier Corten, ‘The ‘Unwilling or Unable’ Test: Has it Been and Could it be, Accepted?’ [2016] Leiden Journal of International Law 777.
  39. Security Council Meeting on the Situation in the Middle East, UN Doc. S/PV.7433, 24 April 2015, 19.
  40. Security Council Meeting on the Maintenance of International Peace and Security, UN Doc. S/PV.7527, 30 September 2015 at 18 and 50 respectively.
  41. Olivier Corten, ‘The ‘Unwilling or Unable’ Test: Has it Been and Could it be, Accepted?’ [2016] Leiden Journal of International Law 789.