This dissertation explores the sociopolitical dimensions of transboundary conservation on the post 9/11 Big Bend U.S.Mexico border, one of the most ecologically complex and biologically diverse arid regions on earth. Six protected areas three in Texas and three in Mexico comprise the Big Bend Transboundary Conservation Network where the states of Texas, Chihuahua, and Coahuila meet. This conservation network contains multiple levels of social and political organization that complicate collaborative conservation as it travels across international, institutional, and cultural boundaries. A processual and relational border theory framework combined with ethnographic data informs this research. I investigate cross-scale institutional interplay at the intersection of homeland security and natural resource management along the U.S. side of the Big Bend border, with analytical attention paid to the relations between the U.S. Border Patrol and Big Bend National and Texas State Parks. I then analyze institutional relations across the border, between natural resource managers in Texas and in Mexico, to identify obstacles to collaborative, binational transboundary conservation. Last, I investigate the relationship between resource use and conservation inside Mexicos rea de Proteccin de Flora y Fauna Can de Santa Elena, with analytical focus given to the contested meanings of natural resource and conservation among cattle ranchers and resource managers. The rebordering of the U.S. border as a militarized security mechanism in the war on terror complicates transboundary conservation in the Big Bend borderlands by enclosing shared geographical space as the authoritative jurisdiction of the U.S. Border Patrol and by erecting policy barriers to international collaboration across the border between resource managers from the U.S. and Mexico. The rebordering of a utilitarian landscape as rea de Proteccin de Flora y Fauna Can de Santa Elena in Mexico has created social tension between resource users and resource managers.