Since the collapse of the French Army in the Franco-Prussian War of 1870, military strategists and officials have recognized the critical importance of mobilization and concentration to the outcome of armed conflicts. This study breaks new ground in the study of military history by providing a reconstruction of the French mobilization in 1870. Thomas Adriance demonstrates that the French defeat was largely due to their inability to match Prussia's mobilization and exploit Prussian concentration blunders. A major contribution to our understanding of the reasons for France's costly defeat in 1870, "The Last Gaiter Button" also sheds new light on the general importance of mobilization and concentration in the conduct of modern warfare.