This book examines diplomatic influence and collective decision-making within the transatlantic security regime, focusing on the four major member states of NATO: France, Germany, the UK, and the United States. Two cases of post-Cold War transatlantic military intervention are examined in which regime member states sought to develop and adopt a single, collective policy on the use of military force outside of NATO's territorial area of operations: Bosnia-Herzegovina and Kosovo. The question is, what conditions or factors increase or decrease the likelihood of the member states of the transatlantic security regime adopting a common, collective policy with regard to military intervention in a given case? The author answers that question by testing the roles of six alternative rival explanations: power, threat perception, international institutions, risk analysis, perceptual lenses, and domestic political pressures.