This dissertation explores the central paradox of Mexico's foreign relations with Cuba and the United States in the decade following the Cuban Revolution--why did a government that cooperated with the CIA and practiced conservative domestic policies defend Castro's communist regime? It uses new sources to prove that historians' previous focus on the foreign and ideological influences on Mexico's relations with Cuba was misplaced, and that the most important factor was fear of the domestic Left. It argues that Mexican leaders capitalized upon their country's "special relationship" with Castro as part of their efforts to maintain control over restive leftist sectors of the Mexican population. This project uses new sources to illuminate how perceptions of threat shaped Mexico's foreign and domestic politics. In 2002, the Mexican government declassified the records of the two most important intelligence organizations--the Department of Federal Security and the Department of Political and Social Investigations. The files contain the information that Mexico's presidents received about potential dangers to their regime. They reveal that Mexican leaders overestimated the centralization, organization, and coordination of leftist groups, and in so doing gave them more influence over policy than their actual numbers or resources logically should have afforded. The dissertation uses the concept of threat perception as an analytic and organizational tool. Each chapter considers a different potential source of danger to the Mexican regime in the context of the Cold War and the country's relations with Cuba. For the sake of clarity, it breaks the threats into the categories of individual, national, and international, even though these subjective categories may blend into one another throughout the course of the analysis. The first chapter begins with an individual threat: Lázaro Cárdenas, a powerful former president who became one of Fidel Castro's most dedicated supporters. The next three chapters analyze threats on the national level by looking at the domestic groups that Mexican leaders perceived to be the greatest dangers to their regime. The final two chapters move to the international level and examine the roles of Cuba and the United States. As a whole, this study of the connections between Mexico's foreign and domestic politics makes a significant and timely contribution to the historiographies of modern Mexico, U.S.-Latin American relations, and the Cold War. / text