This article makes the case for addressing nonhumans as actors in geopolitical processes such as boundary making and enforcement. The challenge of this line of argumentation is to account for nonhumans as actors without enacting dualistic ontologies that locate the natural and social in separate realms. To address this methodological challenge, I present a posthumanist political ecology. I elaborate my argument and methodological approach in relation to my research on the environmental dimensions of U.S. border security. Specifically, I examine how deserts, rivers, Tamaulipan Thornscrub, and cats inflect, disrupt, and obstruct the daily practices of boundary enforcement, leading state actors to call for more funding, infrastructure, boots on the ground, and surveillance technology. As my research illustrates, taking nonhumans seriously as actors alters explanations for the escalation of U.S. enforcement strategies. [ABSTRACT FROM AUTHOR]