On 10 July 1609, a coalition of Roman Catholic states in the Holy Roman Empire gathered together to oppose the Protestant Union which had formed in 1608.
The purpose of the Catholic League, under the leadership of Maximilian I of Bavaria, was to defend Catholic interests in the Empire which seemed pressing particularly after a conflict at the city of Donauwörth in Bavaria on 25 April 1606. The Lutheran majority in Donauwörth forbade a St. Mark procession involving the waving of flags and singing hymns from passing through the town even though this was to be permitted according to the terms of the Peace of Augsburg. This was followed by other anti-Catholic incidents in the Empire.
While the purpose of the League was intended to “promote peace” by being a counter-force to the earlier formed Protestant Union, the establishment of large military forces under the leadership of opposing Catholic and Protestant leadership in fact made war between the two sides more likely. This will be especially seen in the eventual outbreak of the Thirty Years War in 1618 which saw significant fighting between Protestants and Catholics.
Indeed the Thirty Years’ War will bring about the end of the Catholic League as the peace terms will prohibit such inter-state agreements and alliances within the Holy Roman Empire. Only direct alliances between states and the Emperor will be permitted.