Although the United States and Nicaragua maintained cooperative relations for a period after the Sandinista revolution of 1979, ties between the two states deteriorated. What explains the shift from cordial to hostile relations? The two dominant explanations have been that the aggressive policies of the new Reagan administration forced the Sandinista regime into a defensive position of hostility or that a downward spiral occurred based on the security dilemma. This paper rejects both and offers an alternative: The Sandinistas for ideological and domestic political reasons chose antagonistic relations when they opted to arm and support the rebels in El Salvador in violation of an earlier agreement between the two states. Drawing on research that shows the aggressive tendencies of revolutionary states, the article contrasts this theory with the spiral model. Moreover, analysis of the archives of the early Reagan administration shows that it wanted to find an accommodation with Managua. Although the revolutionary Sandinistas were motivated by ideology, Washington was more influenced by geopolitical concerns.