International relations scholars increasingly debate when and how international institutions influence domestic policy. This examination of ethnic politics in four Baltic and East European countries during the 1990s shows how European institutions shaped domestic policy, and why these institutions sometimes failed. Comparing traditional rational choice mechanisms such as membership conditionality with more socialization-based efforts, I argue that conditionality motivated most behavior changes, but that socialization-based efforts often guided them. Furthermore, using new case studies, statistics, and counterfactual analysis, I find that domestic opposition posed far greater obstacles to socialization-based methods than it did to conditionality: when used alone, socialization-based methods rarely changed behavior; when they did, the domestic opposition was usually low and the effect was only moderate. In contrast, incentive-based methods such as membership conditionality were crucial in changing policy: As domestic opposition grew, membership conditionality was not only increasingly necessary to change behavior, but it was also surprisingly effective.