In terms of international law, the militant group which calls itself the Islamic State (IS) naturally poses questions of illegitimacy in the context of the law of belligerency and International humanitarian law (ius in bello). However, the group claims to operate within a distinct and parallel law, i.e. Islamic (international) law and the support it enjoys stems directly from this claim. A focus on public international law alone would thus provide only an external claim to their illegitimacy, one which they and their many supporters would disregard as meaningless, since it could never be above divine commands. In light of this and in light of the fact that in most Muslim majority states, secularism has never obtained the respect it enjoys in the west, it is thus important to ask the questions of the legitimacy of this group, their actions and their political formations from within the norms of Islamic international law. This study therefore essentially aims to provide answers to questions already raised by many scholars and international organisations: What are the justifications for waging war on which the group relies? Are these justifications valid under Islamic international law? Who can declare jihād and under what conditions? Could their political formation rightfully claim to be a Caliphate under Shariʿah? Why have the militants been denied the recognition of their chosen name by the vast majority of Muslims worldwide and have rather been branded with the derogatory acronym Da‘esh or named ‘the modern day Khārijites’? The answers to these questions are crucial because Da‘esh recruitment and rallying narrative relies on depicting their struggle as a just and noble jihād in line with the tenets of Islam. Arguably, this study would also assist any future prosecution of this group. It would help adjudicators in asserting the legitimacy of their judgments, if they were able to prove that such judgments are compatible with the legal and belief systems recognised by the actors at trial.