During the nineteenth century, South America was plagued by internal rebellions that destabilized the region's economies and political systems. At the beginning of the twentieth century, however, levels of political violence throughout the region declined dramatically. Existing scholarship has paid surprisingly little attention to this historic transformation, in part because comprehensive data on revolts have been lacking. Historical narratives and an analysis of a comprehensive new dataset on all revolts in South America from 1830 to 1929 show that the decline in revolts stemmed in large part from the expansion and professionalization of the region's militaries, which were driven by the export boom and the threat of interstate conflict. Nevertheless, not all types of rebellions declined precipitously during this period, as an original typology of revolts shows. Although the strengthening of the region's armed forces discouraged revolts by non-state actors, it did not significantly reduce rebellions from within the state apparatus, such as military coups.