My dissertation is a social history of the Peruvian-Chilean maritime world in the nineteenth and early-twentieth centuries. The primary questions are: what forms of cooperation or solidarity existed between Peruvians and Chileans? And how does an oceanic perspective change terra-centric approaches? These questions challenge a literature based on endless conflict between the two states. I approach these questions through three perspectives: medical communities; maritime and port workers; and policing. I show how cholera’s spread into Chile in late-1886, only three years after the end of the divisive War of the Pacific (1879-1883), led doctors in both countries to work together to confront the epidemic, developing a “science without a nation.” Maritime and port workers in Peru and Chile built transnational bonds of solidarity across the South American littoral and into the Pacific. Laboring alongside people from around the world, and engaging with anarchist politics, they produced and circulated anti-nationalist narratives that competed with nationalist sentiments. Cognizant of the oceanic organizing, each state created new police forces and criminological techniques to surveil these networks. By pulling these threads together, I show how professionals, proletarians, and the police actively forged, on a quotidian level, relations of mutual support through the Pacific World.