The alliance between Prime Ministers Jose Maria Aznar and Tony Blair represented a crucial moment in recent European and World politics, owing to the divisive nature of their support for the United States in the wake of the terrorist attacks on September 11, 2001. The extent of their collaboration, combined with their interventionist approach to tackling global threats, reflected an unprecedented level of cooperation between Spain and the United Kingdom. This book sets out to explain Aznar and Blair's foreign policy and how they came to support the United States. Understanding their relationship is imperative to explaining divisions over European reform, intervention, tackling rogue states, the response to 9/11, and the war on terrorism. This critical period in world politics has been subject to significant academic analysis, but the motivations behind the adoption of a pro-US foreign policy by Aznar and Blair have hitherto not been examined from a comparative perspective. This analysis uncovers factors that have either been overlooked or understated in terms of their influence throughout the Aznar-Blair relationship, particularly regarding their alliance in the European Union before September 2001. This compelling comparative study is invaluable to explaining how two leaders from different political backgrounds and traditions became such close allies, and how they came to support US foreign policy despite the pressures of deep divisions within the EU political bloc. This book is published in association with the Canada Blanch Centre for Contemporary Spanish Studies, LSE.