Recent research on violence against civilians during wars has emphasized war-related factors (such as territorial control or the characteristics of armed groups) over political ones (such as ideological polarization or prewar political competition). Having distinguished between irregular and conventional civil wars and between direct and indirect violence, I theorize on the determinants of direct violence in conventional civil wars. I introduce a new data set of all 1,062 municipalities of Catalonia during the Spanish Civil War (1936–1939) and I show that the degree of direct violence against civilians at the municipal level goes up where prewar electoral competition between rival political factions approaches parity. I also show that, following the first round of violence, war-related factors gain explanatory relevance. In particular, there is a clear endogenous trend whereby subsequent levels of violence are highly correlated with initial levels of violence. In short, the paper demonstrates that an understanding of the determinants of violence requires a theory combining the effect of political cleavages and wartime dynamics.