Insurgency and counterinsurgency are widely described as "population-centric warfare": a competition between military actors over civilian loyalties. Drawing on a high-resolution conflict event data set and a new approach for analyzing reactive behavior in space and time, this article answers the question of how civilian cooperation and defection are systematically driven by incumbent and insurgent violence. Theoretically, the study contributes to resolving a dispute between proponents of deterrence- and alienation-based approaches to population-centric warfare. Empirically, this analysis improves upon the mixed results from previous microstudies in favor of an integrated picture: indiscriminate violence has almost no effect on collaboration with the adversary in its immediate spatiotemporal vicinity. At larger levels of aggregation, however, a clear reactive pattern of collaboration with the adversary becomes visible which is in line with alienation-based reasoning.