In cross-national studies, ethnic exclusion is robustly associated with the onset of violent challenge to incumbent regimes. But significant variation remains at the subnational level—not all members of an excluded ethnic group join in challenge. This article accounts for intra-ethnic group variation in terms of the network properties of local communities, nested within ethnic groups, and the informal ties that regimes forge to some segments of the ethnically excluded population. Mobilization within an excluded ethnic group is most likely among local communities where members are densely linked to one another and lack network access to state-controlled resources. Drawing on a case study of the Syrian city of Homs in the 2011 uprising, this article demonstrates how the Syrian regime’s strategies of managing the Sunni population of Homs shaped patterns of challenge. On the one hand, the state’s toleration of spontaneous settlements on the city’s periphery helped to reproduce dense network ties. On the other hand, the regime’s informal bargains with customary leaders instrumentalized those ties to manage local populations. These bargains could not withstand the regime’s use of violence against challengers, which meant that these same local networks became crucial factors in impelling and sustaining costly antiregime mobilization.