Which factors drive the onset of genocidal violence? While the previous literature identified several important influences, states’ military capabilities for conducting mass-killings and the structure of their security forces have received surprisingly little attention so far. The authors take this shortcoming as a motivation for their research. A theoretical framework is developed, which argues that more differentiated security forces, that is, forces that are composed of a higher number of independent paramilitary and military organizations, are likely to act as a restraint factor in the process leading to state-sponsored mass-killings. Quantitative analyses support the argument for a sample of state-failure years for 1971–2003, and it is also shown that considering a state’s security force structure improves our ability to forecast genocides.