Despite a substantial body of research on foreign policy beliefs and attitudes, we know far less about American attitudes toward U.S. allies and why support varies across individuals. In this paper, we address this gap in the literature, first, by examining variance in individual support for a set of U.S. alliances included in the Chicago Council on Global Affairs survey data and, second, by using experimental data to test how perceptions of threat alter support for U.S. military alliances. We find that perceptions of threat influence support for military allies, increasing support for those allies perceived as directly threatened by U.S. adversaries. In addition, we find that the cultural similarity, isolationism, and type of threat matters. Americans respond differently to threats to allies in regions perceived as culturally similar to the US, depending on whether they are isolationist (or internationalist), and to regional military threats relative to nuclear proliferation. More broadly, Americans appear to update their beliefs both about the relative importance of national security threats and military alliances when given objective threat assessments from national security experts.