Introduction

After 2001, states have increasingly invoked the ‘unwilling or unable’ formula when justifying military intervention against non-state or terrorist targets. Moreover, the closely related term ‘manifestly failing’ has served as a key determinant triggering the international community's responsibility for protecting vulnerable populations following the third pillar of the Responsibility to Protect (R2P) doctrine. Since 2014, the ‘unwilling or unable’ formula has also served to justify US-led air strikes against the so-called Islamic State in Iraq and al-Sham (ISIS) in Syria. Given the formula's apparent rising prominence, this chapter will critically assess the legal foundations and policy practice of the ‘unwilling or unable’ formula and evaluate what this means for the evolution of intervention standards.

The inclusion of R2P in the United Nations World Summit Outcome of 2005 marked a decisive shift in the evolution of interventions for humanitarian purposes. A key trigger of shifting R2P to the level of the international community, and thereby moving towards intervention, is determining that ‘national authorities manifestly fail to protect their populations from genocide, war crimes, ethnic cleansing and crimes against humanity’ (GA 2005: para. 139, emphasis added). The phrase ‘manifest failure’ corresponds to the ‘unwilling or unable’ formula previously used in this context by the R2P-defining International Commission on Intervention and State Sovereignty (ICISS) (ICISS 2001: xi). Apart from this connection, the ‘unable or unwilling’ formula has also been used to justify military intervention in a counterterrorism context. In September 2014, a US-led international coalition has therefore commenced airstrikes on Islamic State in Iraq and al-Sham (ISIS) targets in Syria because ‘the government of the State where the threat is located is unwilling or unable to prevent the use of its territory for such attacks’ (UNSC 2014d, emphasis added). This is the latest prominent example of a state using the ‘unwilling or unable’ formula as a legal justification for military intervention serving the purpose of self-defence against terrorist or non-state actors on the sovereign territory of ‘host’ states.