The Diet of Worms in 1521, presided by Emperor Charles V, declared Martin Luther a heretic and an outlaw (although Charles V was more preoccupied with maintaining his vast empire than with arresting Luther). As a result of Charles V's distractions in East Europe and in Spain, he agreed through the Diet of Speyer in 1526 to allow German princes to effectively decide themselves whether to enforce the Edict of Worms or not, for the time being. After returning to the empire, Charles V attended the Diet of Augsburg in 1530 to order all Protestants in the empire to revert to Catholicism. In response, the Protestant territories in and around Germany formed the Schmalkaldic League to fight against the Catholic Holy Roman Empire. Charles V left again to handle the advance of the Ottoman Turks. He returned in 1547 to launch a military campaign against the Schmalkaldic League and to issue an imperial law requiring all Protestants to return to Catholic practices (with a few superficial concessions to Protestant practices). Warfare ended when Charles V relented in the Peace of Passau (1552) and in the Peace of Augsburg (1555), which formalized the law that the rulers of a land decide its religion.