The nineteenth century opened with the See of Peter vacant as rumors circulated throughout war-torn Europe that the papacy had come to an end. By 1870, however, the First Vatican Council had pronounced the doctrine of papal infallibility, turning the pontiff into the Church’s supreme and infallible authority. From the point of view of intellectual history, this volume tells the story of how political ultramontanism helped to reinvigorate and transform the papacy into a diplomatically effective power that could harness the devotion of millions throughout the globe. The new theology generated debate among Catholics across Europe, leading to schism and excommunication but also to new teachings and ecclesiologies and a role for the pope as international arbitrator.