The Introduction explains the history of Haiti and the Dominican Republic and the evolution of the border between the two from the colonial period through the twentieth century. It focuses on the transformation of the border that began with the US occupation and culminated in the genocidal policy of Trujillo. The Introduction presents the ways in which Trujillo’s government confronted the contradictory problem posed by a population that the state recognized as Dominican citizens, but that the state rejected as ethnically and racially undesireable. It demonstrates that the perpetrators of the 1937 Haitian Massacre understood that they were unleashing violence against their own citizens, and not only immigrants and squatters. It considers the 1937 Haitian Massacre in light of its remarkable absence within the evolving field of genocide studies, as well as its comparative significance in relationship to other twentieth-century histories of anti-Black violence in the Americas.