Violent organizations with extreme ideologies such as the Islamic State in Iraq and Syria are often characterized as lawless, irrational, and therefore difficult or even impossible to predict and understand. Contrary to this conventional wisdom, I argue that many armed extremist groups across history and around the world share a common element: law-like systems of rules that they use to justify violence, regulate the conduct of civilians as well as their own members, and to transform social, political, and economic institutions in ways that will advance their ideological and strategic objectives. In Part I, I lay out a new research agenda for the study of “insurgent legality,” arguing that legal scholars and practitioners should take seriously the internal regulatory and governance systems of violent and criminal non-state actors in order to understand not only how these groups emerge and establish legitimacy in the eyes of their members and supporters, but also how their legitimacy can be undermined. In the case of rebel insurgent groups like the Islamic State, I argue that an insurgent group's establishment of a legal system that is perceived by the local population as relatively more effective and fairer than that of the state it seeks to overthrow or secede from can greatly facilitate its capture and governance of territory. But if, over time, the insurgent group's system of law and governance becomes increasingly flawed by procedural injustice, corruption, discrimination against marginalized groups, increasing taxes without corresponding improvements in rights and services, and arbitrary violence, it will lose local support and become more vulnerable to collapse under pressure from growing internal threats of public resistance and infighting as well as the external threat of military defeat.

In Part II, I test and find support for this theory with qualitative and quantitative data collected over the course of more than two years of field research in Iraq and southern Turkey including: an original household survey of 1,458 residents of the Islamic State's former capital city, Mosul; in-depth interviews with more than two hundred Syrians and Iraqis who lived under the Islamic State's rule; and a dataset of more than 1,500 primary source documents that maps the expansion and eventual retreat of the Islamic State's governance and lawmaking activities across time and space in all twenty-two Syrian districts that the group controlled to some extent between 2013 and 2017. Consistent with my theory, this data demonstrates the importance of legal institutions both for the Islamic State's initial success and the ultimate failure of its state-building project. Part III discusses the lessons learned and broader implications of this research for post-conflict peace-building, transitional justice, and countering the potential resurgence of the Islamic State that are informed not only by my academic research but also by my practical experience working with United Nations agencies in Iraq on these issues. I conclude by suggesting that my theory of insurgent legality can be extended to study the internal regulatory and governance systems of other violent or criminal non-state actors in very different contexts including drug cartels, white supremacist movements, pirates, and ransomware hackers.