When and where do states coercively alter their internal demography? We build a theory that predicts under what conditions states alter the demographic “facts on the ground” by resettling and expelling ethno-national populations. We predict that, under particular scope conditions, states will employ demographic engineering to shore up control over (1) nonnatural frontiers, and (2) areas populated by ethnic minorities who are co-ethnics with elites in a hostile power. We then substantiate our predictions using new subnational data from both China and the USSR. Causally identifying the spatially differential effect of international conflict on demographic engineering via a difference-in-differences design, we find that the Sino-Soviet split (1959–1982) led to a disproportionate increase in the expulsion of ethnic Russians and resettlement of ethnic Han in Chinese border areas lacking a natural border with the USSR, and that resettlement was targeted at areas populated by ethnic Russians. On the Soviet side, we similarly find that the Sino-Soviet split led to a significant increase in expulsion of Chinese and the resettlement of Russians in border areas, and that resettlement was targeted at areas populated by more Chinese. We develop the nascent field of political demography by advancing our theoretical and empirical understanding of when, where, and to whom states seek to effect demographic change. By demonstrating that both ethnic group concentration and dispersion across borders are endogenous to international conflict, our results complicate a large and influential literature linking ethnic demography to conflict.