Doctrines have been a prevalent form of foreign policy in U.S. history. This study seeks to explain their origins by examining the Reagan Doctrine, pledging aid to anticommunist guerillas in the Third World. Based on original research and interviews with numerous individuals in the Reagan administration, the author applies two alternative explanations: realist theory, focusing on the international level of analysis, and elite beliefs theory, focusing on individual political leaders and their beliefs. What he finds is that while each perspective is necessary to explain the Reagan Doctrine, neither is sufficient by itself.