Newly elected Holy Roman Emperor Ferdinand II tried to impose unified Catholicism. Protestant states reacted, believing that the act was in violation of Peace of Augsburg, forming the Protestant Union.
Bohemians deposed Ferdinand II and replaced him with Frederick V, Calvinist Elector Palatine. Ferdinand, along with Maximilian of Bavaria and George I of Saxony, crushed the rebellion.
Lutheran king Christian IV of Denmark wanted to expand Danish influence and joined the Protestant cause. Maximilian of Bavaria quickly crushed the opposition, scaring Ferdinand II with his example of military might.
Gustavus Adolphus II of Sweden became Protestant leader. Adolphus was successful in leading the league until he was assassinated by Wallenstein.
The French, under direction of Cardinal Richelieu, wanted to keep Habsburg influence at bay, and continued efforts against Ferdinand II.