The Thirty Years' War (1618–48) occurred during the reign of Elector John George (1611–56). In this struggle, the Elector was at first neutral, and for a long time he would not listen to the overtures of King Gustavus Adolphus of Sweden. Not until the Imperial General Johann Tserclaes of Tilly advanced into Saxony did the Elector join the forces of the Swedish Empire. However, after the 1634 Battle of Nördlingen, the Elector concluded the Peace of Prague with Emperor Ferdinand II in 1635. By this treaty, Saxony received the Margravates of Upper and Lower Lusatia as a Bohemian fiefdom, and the condition of the Church lands that had been secularized was not altered. The Swedes, for their part, took revenge with ten years of plunder.