Protestantism had provided to be the largest threat towards Catholicism for some time, and played a large role in the outbreak of the Thirty Years War. In 1555, the Peace of Augsburg had brought an end to religious warfare between German Catholics and Lutherans, but they continued to contend for control of various principalities. The treaty did not recognize the Calvinists, even though many German states adopted the faith. Elector Palatine Frederick IV assumed leadership in forming a league of German Protestant states named the Protestant Union. Duke Maximilian of Bavaria responded to this by creating the Catholic League, and created fo