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 long-term threat to the state system.

Most wars today are civil wars, but we have little understanding of the conditions under which rebel groups might comply with the laws of war. i ask three questions in this essay: What do the laws of war require of rebels, or armed nonstate actors ( ansa s)? To what extent are rebels aware of the laws of war? Under what conditions do rebel groups comply with international humanitarian law? i argue that the war aims of rebel groups are key to understanding their relationship with the laws of war. In particular, secessionist rebel groups – those that seek a new, independent state – are especially likely to comply with the laws of war as a means to signal their capacity and willingness to be good citizens of the international community to which they seek admission.