Existing categorizations of rebel groups have difficulty classifying some of today’s most vexing rebels-those, such as the Islamic State, that reject the Westphalian state system and depend on an almost entirely religious justification for their cause. Such rebel groups often have unlimited war aims and are unwilling to negotiate with the states whose sovereignty they challenge. In this essay, I present the new category of “religionist rebels.” I show that religionist rebels have been present throughout the history of the state system, and explore the particular challenges they pose in the civil war context. Religionist rebels are often brutal in their methods and prosecute wars that are especially difficult to end. But the nature of religionist rebellion also suggests natural limits. Thus, religionist rebels do not, ultimately, present a longterm threat to the state system.