What are the conditions under which civilians living in territory captured by an armed group will prefer its system of "rebel governance" to that of the incumbent state (Arjona 2016, Mampilly 2011)? Given the opportunity to flee to government-controlled areas, IDP camps, or neighboring countries, why do some people stay? This paper uses the case of the Islamic State’s three-year rule over the Iraqi city of Mosul to explore the individual-level motivations--social, political, economic, and ideological--that influence the decisions of potential migrants to stay or leave territory captured by a state-building armed group. Through a survey of 1,400 residents of Mosul and semi-structured interviews, I compare and contrast the motivations of "stayers" (those who remained in Mosul after IS's arrival in June 2014) and "leavers" (those who fled Mosul in the early days of IS rule and have since returned to the city). While recognizing that decisions to stay or leave are multi-factorial, I develop and test a discrete-choice theory of migration hypothesizing that civilians who had previously experienced high levels of injustice in their interactions with Iraqi state authorities--for example, bribery, sectarian discrimination, and police harassment--were more likely to be "stayers" than those with fewer or no such grievances. If this hypothesis is supported by the data, it would suggest that weak rule of law in Iraq contributed to civilian cooperation with and support for IS. The survey will also assess several alternative explanations that could explain migration decisions: economic resources, social networks, information, threat perceptions, and ideology.