The author tries to trace the complex processes that combined to form the policies of the Roman Curia in cooperation with Propaganda, the Jesuit Fathers, and at times with the Great Catholic Powers vis-à-vis Scandinavia during the reigns of Christian IV of Denmark and Norway and most especially of the warrior king, Gustavus Adolphus of Sweden and of his daughter Queen Christina. On the basis of the original authentic documents in deposits all over Europe, though most especially in the Vatican, Propaganda, and Jesuit archives, the author attempts to unfold the clandestine work of the Counter-Reformation orchestrated from Rome during the Thirty Years' War. Moreover, he provides a partly new study of the motives that brought about the reconciliation with Rome of Queen Christina of Sweden.