This study of United States relations with post-colonial Indonesia sheds light on how prevalent assumptions about and interpretations of a major world religion, Islam, shaped policymakers' attitudes and decisions concerning U.S. engagement with Muslim actors abroad. It examines not only the accuracy and nuance of U.S. officials' knowledge about Islam in Indonesia, but also religion's shifting salience in U.S. foreign relations during the height of the Cold War. It argues that religion mattered in the construction and implementation of U.S. policies toward Indonesia—but under specific, changing conditions, not as an overarching framework. Islam provided one lens by which U.S. officials understood the behavior and goals of the Indonesian government and, perhaps more consequentially, the character of the majority of Indonesia's people. While interpretations of Islam in Indonesia evolved over the course of subsequent U.S. administrations—and the significance and utility of these perceptions waxed and waned over time—in general the U.S. foreign policy establishment tended to see religion as a means to an end, namely, as an instrument that at times proved useful in the struggle against global communism.