Islamic State (IS) is a radical jihadist armed group that controls vast swathes of territory in eastern Syria and across northern and western Iraq. Its existence and activities have prompted numerous States to intervene against it. Rather than focusing on the responsibility of Islamic State itself, the present contribution assesses the legality of the behavior of the States who interact with the armed group. In particular, the authors assess the legality of the use of force against Islamic State, exploring the current international regulation of self-defense and military interventions by invitation of the territorial State. The chapter also analyses whether any State bears responsibility for (or in relation to) the violations of international law performed by Islamic State
