As the UK leaves the European Union and as the multilateral order is increasingly under stress, bilateral security links are more important than ever. Among such relationships, the UK-France partnership has become particularly critical in the past decades. Alice Pannier's Rivals in Arms reveals the history of the growing special partnership between Europe's two leading military powers in the twenty-first century. Using an innovative analytical framework rooted in theories of cooperation and negotiation, this book exposes the challenges the two countries have faced to develop, equip, and employ their military capabilities together. Through a decade-long study, Pannier highlights how France and the UK have endeavoured to make their partnership more effective and resistant to domestic and international shifts, including Brexit. Building on more than one hundred interviews with key stakeholders and unmatched access to primary sources, Rivals in Arms takes the reader behind the scenes, investigating the complicated but crucial defence relationship between France and the UK - a relationship that is critical to the future of Euro-Atlantic security.