This dissertation examines the politics of urban planning in the San Diego-Tijuana borderlands. It covers the period from the 1920s, when Tijuana’s first urban boom set the stage for difficulties in water and wastewater management on both sides of the border, to the 1970s, when cross-border emergency water and sewer lines permanently linked the cities of San Diego and Tijuana. American private enterprise incentivized migration from Mexico’s interior to Tijuana, driving rapid population growth in the Mexican city. This growth outpaced the development of essential water and wastewater infrastructure. When sewer systems failed on the Mexican side of the border, human waste contaminated properties and public places downstream along the Tijuana River in the United States. Likewise, when Tijuana used water from the basin’s underground and surface flows, it diminished the water supplies available for American communities in the river valley. Transborder diplomacy therefore assumed a central role in urban planning and development, and the communities of South San Diego and Tijuana grew increasingly dependent on decisionmakers in Mexico City and Washington. Meanwhile, residents in the borderlands continually sought to maintain their national autonomy through cross-border dialogue between local officials, city planners, civil engineers, and community leaders. But the division of the Tijuana River basin between the two nation-states—a product of the U.S. conquest of northern Mexico in 1848—made urban sustainability and national sovereignty incongruent goals, forcing planners and policymakers to choose between the two. When authorities in San Diego and Tijuana pursued efforts toward maintaining the sustainability of their cities, they forfeited their communities’ rights to national autonomy. When local authorities sought to maximize their national autonomy, they created conditions that threatened the sustainability of their cities.