Abstract
This paper assesses the legality of the use of force against the Islamic State (IS). It first demonstrates that the IS, despite its name, is not a State but, rather, an armed non-state actor.
It then gives a short factual background on the military interventions against the IS and on the regulation applicable to the use of force under current international law. In the third step, the paper discusses, one by one, all the different legal grounds that have been invoked, by intervening States or by scholars, to justify the military interventions against the IS (UN collective actions, intervention by invitation, self-defence and humanitarian intervention).
The paper demonstrates that whereas some of these interventions can be legally justified (the US in Iraq, Russia in Syria), the legality of others (Turkey in Iraq, the US in Syria) is questionable, either because the legal grounds provided for them are not generally accepted as valid, or because the application of these grounds in this case at hand gives rise to doubts.