After World War II, the US-led international security order exhibited substantial regional variation. Explaining this variation has been central to the debate over why is there no nato in Asia. But this debate overlooks the emergence of multilateral security arrangements between the United States and Latin American countries during the same critical juncture. These inter-American institutions are puzzling considering the three factors most commonly used to explain divergence between nato and Asia: burden-sharing, external threats, and collective identity. These conditions fail to explain contemporaneous emergence of inter-American security multilateralism. Although the postwar inter-American system has been characterized as the solidification of US dominance, at the time of its framing, Latin American leaders judged the inter-American system as their best bet for maintaining beneficial US involvement in the Western Hemisphere while reinforcing voice opportunities for weaker states and imposing institutional constraints on US unilateralism. Drawing on multinational archival research, the author advances a historical institutionalist account. Shared historical antecedents of regionalism shaped the range of choices for Latin American and US leaders regarding the desirability and nature of new regional institutions while facilitating institutional change through mechanisms of layering and conversion during this critical juncture.