"The Bulgarians reaped what I sowed, I reaped what the Bulgarians sowed; the Greeks reaped what I sowed, I reaped what the Greeks sowed" is a local refrain that is still popular among the elderly to describe the volatile conditions of life for the inhabitants of Eastern Thrace, Turkey's northwestern borderlands, during the early 20th century. Based on ethnographic fieldwork and multi-sited archival research, this study examines the interwar transformation of Eastern Thrace through the prism of sovereignty and borderland security. The paper begins with a description of historical developments, socioeconomic conditions, and structural constraints that undermined state sovereignty and borderland security in Thrace after years of war that spanned the Balkan Wars, World War I, and the Turkish War of Independence. Chief among these structural constraints were population loss, agrarian crisis, and the demilitarized zones (DMZs) that the postwar Treaty of Lausanne had introduced, one along Turkey's border with Greece and Bulgaria and another around the Turkish Straits. The paper then documents how the Turkish political and military elites came into a particular geopolitical consciousness about Thrace in the 1930s, viewing the region as a vulnerable yet indispensable frontier due to its geographical, symbolic, and military significance. In their quest to re-border Thrace to extend state sovereignty and ensure borderland security, the elites combined the tools of international diplomacy with a regional policy that sought to repopulate, redevelop, and refortify Thrace. The paper coins the concept of marshaling development to situate these efforts to interrelated civilian and military ends, a daring enterprise that foresaw no less than the complete reordering of peoples, materials, infrastructures, resources, and affective dispositions across the borderland space with a view to the joint goals of defense and development. In conclusion, the paper discusses the continued socioeconomic, demographic, and infrastructural legacies of this interwar past in contemporary Thrace.