(1500–58), Holy Roman emperor (1519–58), and, as Charles I, king of Spain (1516– 56), who fought a losing battle to keep his Roman Catholic empire together in the face of emergent Protestantism and outside pressure.

Charles was the son of Philip I, king of Castile, and Joanna the Mad (1479–1555); maternal grandson of Ferdinand V of Castile and Isabella I; paternal grandson of the Habsburg Holy Roman emperor Maximilian I; and great-grandson of Charles the Bold, duke of Burgundy. He was born in Ghent (now in Belgium) on Feb. 24, 1500. On the death of his father in 1506, Charles inherited the Burgundian realm; following the death of Ferdinand in 1516, he became ruler of the vast Spanish kingdom; and when Maximilian died in 1519, he gained the Habsburg lands in central Europe, where his younger brother, Ferdinand, later Emperor Ferdinand I, was governor. Also in 1519, Charles, having bribed the electors, was designated Holy Roman emperor; he was crowned king of Germany in Aix-la-Chapelle (now Aachen, Germany), on Oct. 23, 1520.

Charles was now by far the most powerful sovereign in Christendom. His inherited lands far exceeded those of the Frankish emperor Charlemagne. His territory included the Spanish kingdoms of Aragón and Castile; the Netherlands; the Italian states of Naples, Sicily, and Sardinia; Spanish conquests in America and Africa; and the Habsburg lands. He ascended the imperial throne at a time when Germany was agitated by Martin Luther. In an unsuccessful attempt to restore tranquillity, a great diet was held in Worms in 1521, before which Luther made a memorable defense of his doctrines. The diet rejected his position, and Charles subsequently issued an edict condemning Luther. At this time rivalry between France and Spain over the Italian lands and Burgundy led King Francis I of France to take up arms against Charles, whose attention was drawn away from Germany's internal affairs.

The war between Charles and Francis, in which Charles was allied with Henry VIII of England and the powerful Charles, duke of Bourbon (1490–1527), proved disastrous to France. Francis was taken prisoner in 1525, when the French were defeated at Pavia (near Milan, Italy). In January 1526 he was forced to sign the Treaty of Madrid, relinquishing his claim to Italy and abandoning Burgundy. Soon after his release the following year, Francis renewed the struggle, now aided by Henry VIII and Pope Clement VII, who was anxious to rid Italy of the imperial armies. The pope was captured at Rome in 1527 and was kept captive for seven months. The war ended with the signing by Charles and Francis of the Peace of Cambrai in 1529. Francis again renounced the Italian lands, and Charles ceded Burgundy to France. In 1530 the pope crowned the victorious monarch in Bologna as Holy Roman emperor, the last coronation of a German emperor by the pope.

Charles had been anxious to end the war with the French so that he could put down the religious revolt in Germany and prevent the Ottoman Turks from overrunning Europe. The Turks controlled the Balkan Peninsula, and in 1526, the year that Ferdinand I laid claim to the Hungarian throne, Sultan Suleiman swept over Hungary. Three years later the Turks laid siege to Vienna. In 1535 the Genoese admiral Andrea Doria, in the service of Charles, led an expedition to Africa, defeated the Turks at Tunis, and freed about 20,000 Christian slaves. In 1538 Charles formed an anti-Turkish alliance with Pope Paul III and the city-state of Venice. The alliance was unsuccessful, and in 1547 Ferdinand signed a 5-year treaty with the Turks.

The failure of Charles to repel the Turks resulted in part from his inability to bring religious peace to his empire, particularly Germany. The spread of disorder during the Reformation emboldened the German princes to seek autonomy for their states. The peasants took advantage of the turmoil in 1524 and revolted. In 1530, shortly after his coronation, Charles convoked a diet in Augsburg to discuss the religious problem. The Protestant princes stated their creed in the Augsburg Confession, which was unacceptable to Charles. Negotiations thereafter failed, and in 1531 the princes formed the Schmalkaldic League. The domestic unrest and the continued war with the Turks forced the emperor to postpone his suppression of the Protestants and to grant them some liberties in 1532 in the Peace of Nuremberg.

In 1536 Charles was again at war with France. The war was terminated by the Treaty of Nice in 1538, granting Francis most of the Piedmont region of Italy. The war was resumed in 1542 and ended in 1544 by the Treaty of Crépy, which largely reaffirmed the earlier Peace of Cambrai. Charles, no longer fighting the French or Turks, turned his attention to the princes and the city-states of the Schmalkaldic League. In 1546 the emperor moved against the southern German principalities, and at Mühlberg, Saxony, on April 24, 1547, he scored a decisive victory against the Protestants. His success was temporary; in 1551 Magdeburg, a great stronghold of Protestantism, fell to Maurice, duke of Saxony (1521–53), but Maurice, who had previously supported the emperor, suddenly deserted Charles, allying himself with King Henry II of France. Charles fled before the Protestants. In 1552, through his brother Ferdinand, he concluded the Peace of Passau, by which the Lutheran states were allowed the exercise of their religion. In 1555 the settlement was reaffirmed in the Peace of Augsburg. Meanwhile, in 1552, Henry II had seized the bishoprics of Toul, Metz, and Verdun, and an attempt by the emperor to reconquer Metz failed.

Weary of the constant struggles and heavy responsibilities of his scattered realms, Charles in 1555 resigned the Netherlands and, in 1556, Spain, to his son Philip II. In 1558 Charles abdicated the imperial crown in favor of his brother, Ferdinand I, and retired to the monastery of San Jerónimo de Yuste in Estremadura, Spain. He died there on Sept. 21, 1558.

For further information on this person see the section Charles V (Holy Roman Empire)