Taking up Lynn White, Jr.’s argument that Christianity is largely responsible for contemporary ecological crises, this chapter develops an environmental historical reading of the Christian just war tradition’s transition from its late medieval into its early modern forms. That reading reveals not only the flaws in White’s argument but the many ways that the nonhuman natural world was understood by late medieval just war thinkers, including as resource, brake, enemy, and collection of signs. Attending to the environmental conditions and human interactions with the nonhuman natural world that shaped late medieval Europe and gave rise to early modern projects of colonialization and conquest helps to clarify the range of forces at work in shaping just war thinking and modernity. Among the implications of an environmental historical reading of the history of Christian just war, thinking is not only a recognition of the ways that the natural and the political interact but the need for a richer vocabulary to express those interactions in a time of growing climate-shaped violence.