This article represents an initial foray into the global environmental history of the First World War and suggests new approaches that can change our understanding of the conflict. With ravaged farmlands, charred trees, and muddy quagmires as iconic images of the First World War, scholars have generally tended to overlook the place and the role of nature. Yet only by taking the environment into account can we fully understand the trauma of war and how this conflict in particular shaped the most basic levels of human existence for years to come. Armies in the First World War were both social and biological entities, which depended on a “military ecology” of energy extraction, production, and supply. To keep soldiers and machines in action, belligerent states commandeered food and fuel throughout the biosphere, extending the war's environmental reach far beyond the western front. Examining a number of the ways that war shaped the periphery—evolving disease ecologies in colonial Africa, tin extraction in Southeast Asia, and food production in Latin America—will show that the boundaries of belligerency were vast. These three regions also illustrate the different ways in which the preparation and pursuit of war transformed societies and the natural world. Seeing what George Kennan called the twentieth century's “seminal catastrophe” from an environmental perspective illuminates the global dimensions of the First World War. The conflict accelerated environmental change that had begun in the previous century and established the patterns of military-industrial production, human victimization, and environmental exploitation that defined the twentieth century.