Cabeza Prieta National Wildlife Refuge is located in southern Arizona along the US-Mexico border. Since its inception as a conservation site, the federally-protected wildlife refuge has also been home to a constellation of military activities including an Air Force training and bombing range and, more recently, Border Patrol and the development of Homeland Security. This article describes how conservation and militarization have co-evolved in a complex yet often symbiotic relationship across time and space. Cabeza Prieta’s location on the international border results in an uneasy balance, promoting protection of nature along with protection of national security. The environmental history of the dual processes of conservation and militarization at the refuge enhances our understanding of contemporary environmental challenges in this hybrid landscape. The overarching theme is one of increasing control over a wilderness borderland region—a “disciplined space” in the words of Michel Foucault (2007).