How do international commitments influence domestic political party competition? While scholars have extensively examined how international affairs affect domestic politics through leaders and elites, the effect of international commitments on political party dynamics remain underexplored. In this article, we argue that alliance commitments constrain parties' abilities to credibly differentiate themselves on defense issues by creating domestic and international audience costs for deviating from supporting security partners. Given these constraints, parties reduce their emphasis on defense issues and exhibit less variation in their foreign policy positions. Using data from the Comparative Manifesto Project covering 41 countries between 1991 and 2023, this study shows that NATO and Partnership for Peace membership is associated with decreased emphasis on defense issues during election campaigns. Subsequent results demonstrate that this lack of attention is driven by convergence, as parties in member countries exhibit significantly less variation in defense positions compared to non-member countries.