A common explanation for the increasing polarization in contemporary American foreign policy is the absence of external threat. I identify two mechanisms through which threats could reduce polarization: by revealing information about an adversary that elicits a bipartisan response from policymakers (information mechanism) and by heightening the salience of national relative to partisan identity (identity mechanism). To evaluate the information mechanism, study 1 uses computational text analysis of congressional speeches to explore whether security threats reduce partisanship in attitudes toward foreign adversaries. To evaluate the identity mechanism, study 2 uses public opinion polls to assess whether threats reduce affective polarization among the public. Study 3 tests both mechanisms in a survey experiment that heightens a security threat from China. I find that the external threat hypothesis has limited ability to explain either polarization in US foreign policy or affective polarization among the American public. Instead, responses to external threats reflect the domestic political environment in which they are introduced. The findings cast doubt on predictions that new foreign threats will inherently create partisan unity.