Abstract: This article is about rhetorical framing and its effects on foreign policy outcomes – specifically in intra-alliance relations. It argues that leaders’ attempts to change the framing of existing security concepts alter the context – and cost – of alliance cooperation. In particular, I highlight the mechanism of rhetorical entrapment as the causal link between initial rhetorical action and the changed context of alliance cooperation. While previous studies of rhetorical entrapment have focused on individual-level reputational costs – such as moral shaming or political backlash when hypocrisy is exposed – I focus on the socially constructed nature of political rhetoric and the consequences of language use. That is, I explore why leaders are compelled to choose certain security rhetoric in the first place and how social resonance and audience receptivity can present unintended political constraints and hidden costs. In this way, the findings from this article contribute to two separate bodies of work in the field of International Relations that have yet to be examined closely in tandem: the role of foreign policy rhetoric employed by leaders as part of their political legitimation strategies, and the domestic politics of alliance cooperation. Through comparative case studies of Japan and South Korea prior to and during the early stages of the Iraq War, I demonstrate the role of rhetorical entrapment in explaining the politics of alliance cooperation.