In explaining why states or armed groups employ extreme violence, Jonathan Leader Maynard questions the sufficiency of dominant rationalist accounts and argues for ideology's central role. He rejects associations of ideology with revolutionary fanaticism, arguing that the key ideological foundations of mass killing are radical reinterpretations of conventional ideas about security. This ambitious and elegantly written book not only offers a fresh conceptualization of ideology, but also demonstrates through careful comparative historical analysis how ideologies shape the goals, organization, and legitimation of mass killing. It is essential reading for all those interested in understanding and preventing atrocity crimes.