The German Catholic League (German: Katholische Liga) was initially a loose confederation of Roman Catholic German states formed on July 10, 1609 to counteract the Protestant Union (formed 160

, whereby the participating states concluded an alliance "for the defence of the Catholic religion and peace within the Empire." Modeled loosely on the more intransigent ultra-Catholic French Catholic League (1576), the German Catholic league initially acted politically to negotiate issues with the slightly older Protestant Union.
Nevertheless, the league's founding, as had the founding of the Protestant Union, further exacerbated long standing tensions between the Protestant reformers and the members of the Catholic Church which thereafter began ratcheting upwards with ever more frequent episodes of civil disobedience, repression, and retaliations that would eventually ignite into the first phase of the Thirty Years' War roughly a decade later with the act of rebellion and calculated insult known as the Second Defenestration of Prague on 23 May 1618. Both the Protestant Union and the Catholic League were symptomatic of the eras increasingly intolerant behaviour towards others' personal religious/political beliefs.
The league's army fought and defeated the Danish on August 26–27, 1626 at the Battle of Lutter, destroying more than half the fleeing Danish army. Because this and other victories by Wallenstein, Denmark was forced to sue for peace at the Treaty of Lübeck. Now, the Catholic League hit its peak. Almost the whole German territories were under their control. The danger of imperial hegemony, resulting from this success, made the Swedish King Gustavus Adolphus become involved in the conflict in 1630.