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. Exploring the case of former Yugoslavia, the genocides in Rwanda and Darfur, and the ongoing violence in Syria, Kate Ferguson exposes the relationships between paramilitaries, state commands, local communities, and organized crime. She presents these 'architectures of violence' as a way of comprehending how the various structures of command and control fit together into domestic and international webs of support enabling and encouraging irregular and paramilitary violence. 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.