Schmalkaldic League, a defensive alliance of German Protestant principalities and cities formed during the Reformation. The league was organized in 1531 at the ancient town of Schmalkalden (also spelled Smalkald), Germany, by Lutheran princes and delegates from free cities, after Holy Roman Emperor Charles V had issued an ultimatum calling for unquestioned acceptance of Roman Catholicism. The league raised an army, but was defeated by imperial forces in the Schmalkaldic War, 1546–47. Sporadic armed resistance to the emperor continued, and in 1552 Catholic princes opposed to the growth of imperial power sided with the German Lutherans. In 1555 Charles, bitter and in ill health, acceded to the Peace of Augsburg, which granted each prince the right to choose the religion for his domain.