Under what conditions can UN military peacekeeping operations (PKOs) succeed in contexts of civil war? This is an important question given the prevalence and cost of civil wars and the high, yet not always fulfilled, expectations of very costly military PKOs as responses to them by the international community. Yet, the academic and policy debates on this question are as long-standing as they are unresolved. Our article contributes to existing scholarship in several ways. First, adopting a nuanced and multi-dimensional definition of success that considers violence, displacement, and contagion as its 3 essential components, we identified 19 cases of full or partial successes, and 13 full or partial failures, covering all 32 UN military PKOs deployed to civil war settings. Second, we develop an original dataset and analytical framework that identifies a wide range of plausible factors related to the dynamics of both the intervention and the underlying conflict it is meant to address. Third, applying qualitative comparative analysis to our dataset of these 32 military PKOs, our key finding is that what matters most and consistently across all of these missions is the presence or absence of domestic consent to, and cooperation with, deployed PKOs.