This paper develops a new framework for understanding when and how migrants shape culture, applying it to the Confederate diaspora—a small migrant group that left a large cultural imprint. Southern Whites that migrated after the Civil War played a pivotal role in spreading Confederate symbols and racial norms across the United States by the early 20th century. Their far-reaching influence stemmed from two key conditions: (i) an ideological intensity rooted in their experiences of slavery, secession, and military defeat, and (ii) access to malleable power structures during westward expansion and postwar reconciliation. These conditions enabled them to transmit Confederate culture to both kin and non-Southern neighbors and to expand their reach by mobilizing civil society organization and leveraging positions of authority. They shaped policies and institutions that helped entrench racial norms and inequalities in labor markets, housing, and the criminal justice system. Our findings provide empirical foundations for understanding how migrants can transform local culture, rather than merely assimilate.