Born in 1500, Charles I of Spain was successor of the Austrian Habsburg dynasty and ruled the majority of Europe during the Reformation as Emperor Charles V. On the side of his father, Philip of Burgundy, were the Habsburg Austrian Maximilian and Mary of Burgundy. On the side of his mother, Joana “The Mad” of Castile, were Ferdinand and Isabella, who had united the Spanish crowns of Aragon and Castile. This made Charles the heir to numerous lands that he began to inherit at the age of sixteen. Raised in French Burgundy, his first language was French and he was steeped in the diplomacy of Burgundy politics. Adrian of Utrecht, who would briefly become pope in 1522 before dying a year later, was a member of his court. Between 1516 and the death of his emperor father in 1519, Charles successively inherited the Austrian duchies of Austria, Carinthia, Moravia, Tyrol, and Styria; the Netherlands along with France-Comte from the Burgundy line; and Spain and the Spanish territories abroad, including Milan, Naples, Sicily, Sardinia, and the growing expansion of the New World in Central and South America. The massive land and financial holdings associated with this expansive empire led to grandiose visions of political domination on his part that would result in conflict throughout Europe.

The final step for Charles was election as Holy Roman Emperor over the German territories and the imperial free cities. When Charles’ father Maximilian died, the then-King of Spain was the most likely candidate. While Henry VIII of England and Frederick the Wise of Saxony were early competitors, Francis I of France emerged as the primary opposition. Francis was of the French Valois line that had been at odds with the Austrian Habsburgs. The papacy supported Francis due to growing monarchy of the Habsburgs and its encroachment onto Italian soil, but Charles had something the fiscally strapped papacy did not: recourse to immense funds, including the backing of the Fugger House of Augsburg, to bribe the electors into selecting him. The cost to Charles was 850,000 florins, 500,000 of which were subsidized with a loan from the Fuggers. Charles I of Spain was elected emperor in June 1519 and coronated Charles V in October 1520 at Aix-la-Chapelle, an imperial free city in Germany. He would receive his imperial crown from Pope Clement VII in 1520 at Bologna, the last emperor to be so crowned by a pope.

There were three challenges facing Charles upon taking the mantle of emperor: rivalry with the French Valois, war with the Turks on the eastern edge of the Empire, and the growing Protestant movement. The first two were intertwined and distracted him from addressing the third. The Habsburg-Valois rivalry commanded the most attention during the first decade of his reign. Intermittent war with France led to Charles’ defeat of Francis in 1525. He took the French king prisoner and forced him to sign the 1526 Peace of Madrid, which surrendered French lands in Burgundy and Italy and included an agreement with Francis to marry Charles’ sister, Eleanor. Once back on Spanish soil, Francis rejected the pact claiming it had come under duress and renewed war against imperial troops. Throughout the Habsburg-Valois wars, the papacy had remained on the side of Francis out of fear of Charles’ excursions onto Italian soil, leading Clement VII to join the defensive  League of Cognac with France, Venice, Florence, and Mila. In response, Charles conscripted imperial troops to Italy, where they eventually sacked Rome in 1527—on their own initiative, not the emperor’s—and virtually imprisoned the pope until 1528. The Peace of Cambrai in 1529 put an end to the episode so France and the Empire could address more pressing issues, primarily war with the Turks. France would later break with Charles again to make an alliance with the Turks, but eventually reversed course and in 1544 forged the Peace of Crépy with the Empire to stand as a united front against the Turks, in exchange for either Milan or the Netherlands and this time the marriage of a son of Charles to Francis’ daughter.

The complexities of the Habsburg-Valois contest and the Turkish front distracted Charles from what would become the defining feature of his reign: the growth of Protestantism. The emperor had been crowned for only three months when the fateful Diet of Worms convened in January 1521. At Worms, Luther made his famous confession before the emperor that he would not recant of his writings and was subsequently hidden at Frederick the Wise’s behest in the Wartburg Castle. In the meantime, Charles executed the Edict of Worms making Luther an outlaw in the empire and proscribing all public teaching of his views. The Edict of Worms, however, evoked criticism from Rome. Papal legate Jerome Aleander questioned why an imperial edict was needed of Luther had already been subject to the ban by verdict of the papal bull of excommunication drafted earlier in 1521, Decet Romanem Pontificem. This only served to increase tensions with Rome politically, even though Charles considered himself a devout Catholic and supporter of the papacy who took it as a personal goal to protect church orthodoxy against Luther.

One further complicating factor in Charles’ relationship to the Protestant Reformation was the hesitancy on the part of Rome to call a council. It was never the emperor’s aim to circumvent the ecclesiastical system, but he had anticipated a general council that would rule on Luther’s doctrine and bring peace to the church. Consequently, the 1526 Diet of Speyer passed the recess of the Edict of Worms allowing all princes and cities in the empire to practice religion as they saw fit pending a general council to resolve the theological questions. The 1529 Diet of Speyer revoked the recess and led to the 1530 Diet of Augsburg, where the German princes presented the Lutheran Augsburg Confession and the southern Germans their Tetropolitana, after which the Catholic contingent responded with a hastily written reply of the Roman Confutation. Charles accepted the Confutation and promised to enforce the prohibition of Lutheran doctrine as protector of the church. Nevertheless, he still oversaw several attempts at reconciliation between the two parties over the years. The first came at Augsburg itself following the public presentation of the confessions. Then later, from 1539 to 1541, Charles directed a series of three colloquies in Hagenau, Worms, and Regensburg to reach agreement between Catholics and Protestants in the empire, all with limited results and no lasting impact. Repeated papal promises of a council led him to support the conciliar solution, which finally came to realization with the 1543 convocation of Trent. The actual convening of the council was finally made possible after the 1544 Peace of Crépy brought truce to the war with France and combined support of both Habsburg and Valois rulers for the Tridentine assembly.

After the first sessions of Trent closed, Charles made one final attempt at restoring religious peace within the empire. The 1548 Augsburg Interim gave small concessions to the Protestants, such as clerical marriage and communion in both kinds, but left much of Catholic doctrine largely unchanged. Charles then sought to impose it on the empire until a general council could enforce it more broadly, though numerous German territories vigorously rejected the measures. The religious conflict under Charles’ watch was not resolved until the 1555 Peace of Augsburg, which established the right of both Lutheran and Catholic territories to choose which religion they would follow. The agreement itself was brokered by Ferdinand, the emperor’s younger brother, to whom he had deputed all governing affairs in Germany by 1553.

Charles progressively abdicated his position until finally stepping down in 1556. His younger brother, Ferdinand, succeeded him as emperor. Ferdinand had governed Habsburg lands in Germany and Austria since 1521 and the Hungary and Bohemia since 1526 and had also been named King of the Romans, an honorific designating him as next pope over Charles’ own son Philip II. Philip instead took over the Spanish and Burgundy territories of his father. Charles would spend the remaining two years of his life in Spain, at a villa near the monastery of St. Juste. He died in September 1558.