This book focuses on military cooperation between the United States and its allies in times of war and asks: why are allies so unpredictable? It shows that alliance demands related to military cooperation cannot always be fulfilled by democratic allies due to domestic political constraints.Additionally, concerns over military feasibility can further constrain governments in committing resources to war. Taking these constraints into account is key to explaining the varying levels of military cooperation between democratic allies. This book also explains how American allies can turn down US requests for political and military support. It shows how these allies can deploy strategies to overcome power asymmetries by resorting to leveraging, hedging, and compensation tactics. While they can rarely influence their dominantalliance partner decisively, they are successful in resisting pressures exerted by the US. The argument builds on three central claims. First, power asymmetries between allies translate into different assessments of international threats. Second, when disagreements over threats arise, the outcome ofintra-alliance bargaining is not necessarily dictated by the stronger power. Third, secondary states face a policy trade-off between establishing a reputation as a reliable alliance partner and pursuing politically-safer alternatives. The empirical focus of the book is on formal US allies, as they have the most to gain or lose by going along with American plans. The case studies are the United Kingdom, Canada, and Australia's response to the war in Afghanistan and the war in Iraq during the period from 2001 to 2003.