Under what conditions does a common threat cause rival states to achieve rapprochement? To inform not only contemporary policy debates about coalition-building against terrorism and insurgency, but also theoretical debates about obstacles to cooperation and conflict resolution, this article examines the pattern of rivalry and rapprochement among Central American states—non-Communist allies under the Rio Treaty—in the shadow of the Cuban Revolution of 1959. Given a common threat, why did Honduras and Nicaragua shift from rivalry to rapprochement while Costa Rica and Nicaragua, and El Salvador and Honduras, did not? Drawing on rarely explored published primary sources in English and Spanish, the article argues that the major obstacle to cooperation was the parochial interest of the armed forces in perpetuating the old mission of international rivalry despite the attractive new mission of internal security, and that presidents were able to achieve rapprochement only where state resource constraints compelled...