Paramilitary or irregular units have been involved in practically every case of identity-based mass violence in the modern world, but detailed analysis of these dynamics is rare. Through exploring the case of former Yugoslavia, Kate Ferguson exposes the relationships between paramilitaries, state commands, local communities, and organised crime present in modern mass atrocities, from Rwanda and Darfur to Syria and Myanmar. Visible paramilitary participation in modern mass atrocities has succeeded in masking the continued dominance of the state in a number of violent crises. Irregular combatants have participated so significantly in committing atrocity crimes because political elites benefit from using unconventional forces to fulfil ambitions that violate international law--and international policy responses are hindered when responsibility for violence is ambiguous. Ferguson's inquiry into these overlooked dynamics of mass violence unveils substantial loopholes in current atrocity prevention architecture. Until these are addressed, state authorities will likely continue to use irregular combatants as perpetrators of atrocity.