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 1608), 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 Roman Catholics 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 Catholic Leagues were symptomatic of the era's increasingly intolerant behavior towards others' personal beliefs.
By the time of Grantville's arrival in 1631 Germany, the German Catholic League had control over most of the Germanies and was at war with Gustavus Adolphus and his Protestant allies. Concurrent with Gustavus's entry, the League under Count Tilly had infamously laid siege to the city of Magdeburg, as the city had promised to support Sweden.
Thanks to Grantville's presence and the creation of the New United States, other smaller incursions of the Catholic League had been fought off. The League was later devastated at the Battle of Breitenfeld, as in the original timeline. After Grantville made an amicable alliance with Gustavus, the combined forces helped rout the League at the Battle of Rain. During the battle, Tilly was fatally wounded, as he was in the original timeline. This was a devastating blow to the League.