Taking as its point of departure the perennial tension between particular and universal ethics in international society, this book seeks to explore and understand the motivations of actors in different international contexts where national interests and solidarity concerns intersect. Focusing on a range of regional cases, the book evaluates the respective weight of national interest and internationalist (solidarity) considerations. Ultimately, while classical national interest considerations remain to this day a powerful motivation for power projection, the book shows how an enlightened conception of national interest can encompass solidarity concerns, and how such a balancing of the imperatives of both national interest and solidarity is the major challenge facing decision-makers.