Exploring case studies from the first Gulf War to the Syria crisis, this book discusses different approaches to the use of international law and the role it plays in international power politics. Analysis of the post-Cold War overseas military involvements of Western powers has focused on their legality and legitimacy, allowing for a conflation of the concepts and distracting from the true source of international legitimacy. Demonstrating compliance with international law can be helpful, but it plays a secondary role to other, more powerful considerations such as national interest and shared national security concerns. Exploring the key drivers for decision-makers, this book identifies the impact of previous experience on the use of international law in the quest for legitimacy ahead of launching military action. Patterns in approach and of relations between close Western allies (in particular the UK and US) are identified, offering valuable lessons for future strategic decision-making.This book will appeal to scholars and students of International Relations and International Law. Think Tanks focussing on International Relations and the use of force and practitioners working in the realm of foreign policy with a focus on the UN and international law will also be interested in the study and conclusions drawn.