This book traces the history of Argentine and Chilean pan-Americanism and asks why pan-Americanism came to define inter-American relations in the twentieth century. The Southern Cone and the Origins of Pan America, 1888–1933 offers new perspectives on the origins of the inter-American system and the history of international cooperation in the Americas. Mark J. Petersen chronicles the story of pan-Americanism, a form of regionalism launched by the United States in the 1880s and long associated with U.S. imperial pretensions in the Western hemisphere. The story begins and ends in the Río de la Plata, with Southern Cone actors and Southern Cone agendas at the fore. Incorporating multiple strands of pan-American history, Petersen draws inspiration from interdisciplinary analysis of recent regionalisms and weaves together research from archives in Argentina, Chile, the United States, and Uruguay. The result is a nuanced and comprehensive account of how Southern Cone policy makers used pan-American cooperation as a vehicle for various agendas—personal, national, regional, hemispheric, and global—transforming pan-Americanism from a tool of U.S. interests to a framework for multilateral cooperation that persists to this day. Petersen decenters the story of pan-Americanism and orients the conversation on pan-Americanism toward a more complete understanding of hemispheric cooperation. The book will appeal to students and scholars of inter-American relations, Latin American (especially Chile and Argentina) and U.S. history, Latin American studies, and international relations.