To understand the relationship between religion and genocide in time of war, one needs to distinguish between sacred and secular political religions. Among the genocidal events inspired by political religions based on sacred texts are the St. Bartholomew’s Day Massacre, the Sack of Magdeburg, the British Civil War in Ireland, and Bosnia. I also examine several groups pursuing a genocidal agenda claiming religious justification: al-Qaeda and Islamic State of Iraq and Syria (ISIS). Civil religions and secular political religions discussed are the French Revolution, Italian Fascism, Nazism, and Stalinist Communism. Lacking the restraints found in traditional religions, secular political religion is most dangerous. Large-scale genocides are best explained by diachronic processes entailing subordination followed by gain and then loss by the perpetrators. The presence of loss in various forms is found in virtually all cases. Emotions that typically do not influence routine politics—such as anger and fear—are engaged. All of the cases, even those of minimal loss, are influenced by international events. Without the presence of war, genocides like the Holocaust, and those of the Armenians and Tutsis, are inconceivable. Even as an exclusionary ideology, traditional religion is neither a necessary nor a sufficient condition for all forms of genocide in time of war. But religion can be an enabler that together with other antecedents can lead to genocide. Sacred religious sites can be sensitive locations whose violation inspires violence. Radicalization of religious leaders can occur when their religion appears to be under attack, especially during or following a period of widespread violence.