This article discusses the military campaign against the ‘Islamic State’ (Daesh) in an attempt to illustrate the gaps in the international legal framework that regulates the use of force in dealing with a challenge such as that presented by the Islamic State. This case study was demanding given the need to reconcile state-centred rules with a diverse reality which includes several players, and particularly non-state armed groups in control of territory and population. In order to deal with this issue, the article proposes the invocation of a functional approach, compared with a binary approach, which is suitable in cases where several players exercise power in the same territory. In particular, it suggests that the Islamic State could have been treated functionally as a state for the purposes of self-defence or collective security measures, rather than invoking legal doctrines of unclear status that might result in undermining the international legal system they are invoked to protect.