This paper examines changing German and Korean policies towards transborder coethnics (Germans in Eastern Europe and the former Soviet Union, and Koreans in Japan and China) during the high Cold War and post-Cold War eras. The paper contributes to the emerging literature on transborder forms of membership and belonging by highlighting and explaining the selective, variable, contingent, contested, and revocable nature of states’ embrace of transborder coethnics. The explanation highlights the relationship of transborder populations to predecessor polities; changing geopolitical contexts and domestic political conjunctures; the constitutive, group-making – and group-unmaking – power of state categorization practices; and the enduring institutional legacies and unintended consequences of such practices.