This book offers an accessible and rigorous introduction to the context, diplomacy, and law of the European Union's response to Russia's 2022 invasion of Ukraine. The book explores how the EU responded to the war in the initial months, and is based on research carried out on Russian and European political, diplomatic, and legal texts. Presenting a unique interdisciplinary perspective, the book delves into topics such as the EU diplomatic response, the Ukrainian application for membership of the EU, the policy and legal aspects of EU sanctions against Russia and Belarus. Additionally, the book examines the significance of the EU's unprecedented political response for the constitutional structure of the EU, and for the strategy toward the Russia of the future.