Why, despite their similar goals, do the policy preferences of the European Union and United States diverge on so many multilateral issues? To answer that question, Allies at Odds thoroughly examines recent international efforts in arms control, environmental protection, human rights, and military cooperation. Evidence from twenty separate cases supports the expectations of the realist approach to international politics, which focuses on the role of power above all. Neither cultural factors nor international institutions have as much influence as some expect. This finding was as true during the Clinton Presidency as during the current Bush administration, indicating that focusing on personalities overlooks more substantial and longer-lasting differences between the Atlantic allies.