How can international organizations shape national welfare states? The answer depends on why national governments comply with international organization mandates. International relations scholarship offers two competing compliance models. Enforcement theories emphasize states’ utilitarian calculus and predict that states’ policy preferences determine implementation, while managerial theories attribute non-compliance to states’ capability limitations and emphasize institutional variables. This paper examines the implementation of EU social policy directives through a new quantitative dataset and qualitative case studies of implementation in Greece and Spain. Three proxies for national social policy preferences – low labor costs, high unemployment and early national social legislation – predict implementation delays. At the same time, factors unrelated to national preferences on particular directives have at least as large an impact on timely implementation. Thus, a national bureaucracy’s capacity and the absence of veto players reduce implementation delays. These findings suggest that capabilities influence compliance at least as much as preferences, but through mechanisms different from the ones emphasized in existing work. Although international organizations may not be especially successful in overcoming past policy legacies in favor of future commitments, they can reorient the axes of contestation from left-right to supra-sub national and thus shape national policies.