From 1618 to 1648, Christian princes waged the first pan-European war. Brought about in part by the entrenched passions of the Reformation and Counter-Reformation, the Thirty Years War inevitably drew in the Society of Jesus, or Jesuits, who stood at the vanguard of Catholic Reform. This book investigates for the first time the Jesuits’ role during the war at the four Catholic courts of Vienna, Munich, Paris, and Madrid, and the challenge to the Jesuit superior general in Rome to lead a truly international organization through a period of rising national conflict. War goals varied and changed at the courts as the conflict progressed. Advocates of ‘holy war’ contended with moderates or politiques. As conflicting ideas about the proper relationship between religion and politics came to shift under the pressure of events, it became clear the extent to which the Thirty Years War was a religious war.