In this essay, Michael Goebel examines the historical construction of the Western South Atlantic region--roughly the area consisting of Brazil and Argentina--from the period from 1870 to 1930, during which intensified global exchange fed into the formation of this area as a historical region. In contrast to nationalism and, more recently, globalization and transnationalism, Regionalism remains a relatively unexplored concept by historians. In this essay, Michael Goebel examines the historical construction of the Western South Atlantic region--roughly the area consisting of Brazil and Argentina--from the period from 1870 to 1930, during which intensified global exchange fed into the formation of this area as a historical region. IntroductionMapping a Region1. Population Movements and Socio-Economic Structures2. Wars, Informal Empire,and National/Regional Border-MakingMigrations1. Overseas Immigration2. Intra-Regional, Inter-Regional, and Internal MigrationsDynamics of Nationalization1. Nationalizing Migrations2. Cultural Nationalisms, Regionalisms, and NativismsNationalism and Latin American RegionalismConclusionNotes2013