The dissolution of the Austro-Hungarian Empire in 1918 sent East-Central Europe into chaos, with the Polish and Czechoslovak nation-states clashing over the contested region of Teschen Silesia. Seeking to prevent ethnic violence, the Allies split the region along with its multiethnic urban center, the city of Teschen, between the two nation-states in July 1920. Tracing cross-border cooperation on the local level, this dissertation shows how municipal administrators peacefully transformed the imperial city into a Polish city, Cieszyn, and a Czech city, Český Těšín. Despite rising ethno-nationalism, Polish, Czech, German, Silesian, and Jewish local leaders made the surprising choice to prioritize the basic needs of their constituents by maintaining the economic and social benefits of a unified city. In so doing, nationalist rivals stood side by side as they lobbied for the relaxation of border restrictions to stimulate the local economy, preserved cross-border electrical wiring and water pipes to cut costs, and made compromises in official language policies. Analyzing local, regional, and national administrative documents as well as newspapers, this dissertation argues that local leaders chose to be administrators first and nationalists second. Responding to new borders pragmatically, they actively stabilized the nationally contested territory through cross-border cooperation, often in the face of countervailing pressures from centralizing national governments. In so doing, this example reveals that nationalism alone cannot explain political decision-making in interwar East-Central Europe as tensions between the center and the periphery frequently eclipsed national competition on the local level. Enhancing our understanding of imperial structures’ afterlives by probing the role of local governments in post-conflict reconstruction, this dissertation decenters national leaders and international peacekeepers. Instead, it positions municipal administrators as crucial ‘paradiplomatic’ actors and their small towns as vital arenas for peacebuilding.