After 1530 the emperor, Charles V, made clear his intention to crush the growing heresy. In defense, the Lutheran princes banded together in 1531 in the Schmalkaldic League, and between 1546 and 1555 a sporadic civil war raged. The combatants reached a compromise in the Peace of Augsburg (1555), which allowed each prince to decide the religion of his subjects, forbade all sects of Protestantism other than Lutheranism, and ordered all Catholic bishops to give up their property if they turned Lutheran.
The effects of these provisions on Germany were profound. Lutheranism became a state religion in large portions of the empire. From Germany it spread to Scandinavia. Religious opinions became the private property of the princes, and the individual had to believe what his prince wanted him to believe, be it Lutheran or Catholic.