After the diet of 1530, Charles left Germany for more than a decade, occupied with troubles in the Mediterranean, the Netherlands, and, once again, France. In 1535 he campaigned against Tunis to subdue the Barbary pirates who, as a naval arm of the Ottomans and as corsairs and privateers, had been making navigation unsafe. Renewed war with France was temporarily halted in 1538 by a treaty meant to last 10 years, but in 1542 France struck again, along with several European allies, including the duke of Gelderland and Cleves (or Kleve), whose lands were claimed by Charles as part of his Burgundian inheritance. The emperor’s conquest of this duchy in 1543, which considerably broadened his power base, and the peace he concluded with France in 1544 (the Peace of Crépy), followed by an armistice in 1545 with the Ottoman Empire, left him free at last to deal decisively with the German Protestants.

The emperor’s policy toward religious deviants was guided by his concept of empire. The universal realm over which he hoped to reign faced external and internal threats; its desired unity and order were assaulted by infidels from without and by national rivalries and heresy from within. He had dealt with the first and second threats; now he turned his attention to the third. Protestantism had spread rapidly in Germany. More than a religion, it was, by the 1540s, a full-fledged political movement with a growing military capacity. The number of Protestant territories had recently grown to include, among others, Brandenburg, the Palatinate, Albertine Saxony, and the bishoprics of Cologne, Münster, Osnabrück, Naumburg, and Merseburg. In Philip of Hesse the Lutherans had an able political strategist. At least provisionally, pending the settlement of all religious issues by a general council, the Protestants had won grudging recognition of their right to exist. Such a council was actually summoned by Pope Paul III—though only upon repeated prodding by the emperor—but there were few signs that the Protestant states would submit. In 1545, therefore, Charles decided on war. He found a pretext in the capture, by Lutheran princes, of the duke of Braunschweig-Wolfenbüttel, a Catholic who had tried to reconquer the lands from which he had been expelled by his Lutheran subjects. Claiming that this capture violated imperial law, Charles opened the conflict in 1546, in which he was joined by Maurice, duke of Saxony, an ambitious Lutheran prince to whom Charles had secretly promised the Saxon electorship. The ensuing war fell into two phases, the first of which saw the emperor victorious at the Battle of Mühlberg, in 1547. Capitalizing on this strong position, Charles in 1548 forced the estates to accept an Interim, a temporary religious settlement on the emperor’s terms. It was the political concessions Charles demanded from the estates, however—concessions that would have permanently limited their autonomy—that led to a resumption of war. Among the Protestants the lead was now taken by Maurice of Saxony, who had abandoned the emperor and had obtained material support from the new French king, Henry II, for fighting on the Protestant side. The resulting “Princes’ War” was brief (1552–53) and inconclusive, and in 1555 a peace was signed at an imperial diet held, again, in Augsburg.

The Peace of Augsburg closed one epoch of German history and opened another. It decided the religious issue but did so in a way bound to occasion future problems. It reinforced the princes’ authority over their territories but failed to settle their relations with the emperor. Most important, it legalized Lutheranism, laying down the rule, later epitomized in the phrase cuius regio, eius religio (“he who governs the territory decides its religion”), that each ruler in the empire—i.e., each prince or city government—could opt for either the Roman Catholic or the Lutheran religion (jus reformandi) and that this choice was binding on everyone under that ruler’s jurisdiction. Only one faith could legitimately exist in a given state, and that faith had to be the ruler’s and could be only Catholicism or Lutheranism; Calvinism, Zwinglianism, and Anabaptism were excluded. A subject unwilling to live by this choice was free to emigrate and take his belongings with him (a provision considered liberal at the time). Confiscated church properties could be kept by the governments that had taken them. An Ecclesiastical Reservation prevented ruling prelates from converting their lands along with them. These terms make it clear that the real winners of the war, and of the entire Reformation period, were the territorial princes, whose authority and power, which now encompassed the church, were greatly increased. As for the emperor, he abdicated in frustration and retired to a monastery in Spain, leaving his Spanish and Burgundian crowns to his son Philip and the empire and the Habsburg lands in central Europe to his brother Ferdinand. These two men, as Philip II and Ferdinand I, strong-minded Catholics both, were to play prominent roles in the period of Counter-Reformation and confessionalism that dominated Europe after 1555.