The European Union has made remarkable, unprecedented advances in both external and internal security integration. "Epistemic communities" of ambassadors, military generals, scientists, and other experts who supersede national governments in the diplomacy of security decision making are taking advantage of their shared expertise, common culture, professional norms, and frequent meetings to forge new levels of collaboration. Mai'a K. Davis Cross brings together numerous personal interviews and recent government documents across five separate case studies to construct a microsociological account of how EU governance really works and what future role the EU is likely to play internationally.