This paper examines the various legal grounds on which States may justify the use of force against ISIL in Syria and Iraq. As a preliminary question to this analysis, the paper elaborates on whether ISIL can be regarded as a State under international law. It identifies two novel arguments for refuting ISIL's statehood besides the regularly cited Montevideo criteria. First, ISIL intends to become a worldwide Caliphate instead of seeking statehood. Second, the very existence of ISIL as a State would violate jus cogens, and States are barred from recognizing entities that were formed in breach of peremptory norms.
The paper goes on to examine the different legal justifications that the intervening States invoked or could have invoked to preclude violating Article 2(4) of the UN Charter. The paper identifies and assesses four such grounds, namely: (i) intervention by invitation, (ii) Security Council authorization, (iii) the right to individual or collective self-defense, and (iv) humanitarian intervention. The paper provides an in-depth survey of State practice and opinio juris on the "unable and/or unwilling" test, and analyzes the anomalies of this novel and vague doctrine. As an overall framework, the paper suggests the "war on ISIL" is better described as a legal rather than military battlefield.
Although the battle over geopolitical dominance overshadows other aspects of the conflict, scholarly debates concerning the scope of Article 2(4), the criteria for issuing a valid invitation, the binding nature of a Security Council decision, the well-founded nature of the unable and/or unwilling doctrine, and the possibility of a humanitarian intervention will have even more far-reaching consequences, as certain doctrines and their infiltration into state practice can hollow out the entire jus contra bellum system. The paper concludes with a workable proposal to use force against ISIL, and non-state actors in general, within the current confines of the UN Charter.
