Conflicts with the Ottoman Empire

Charles fought continually with the Ottoman Empire and its sultan, Suleiman the Magnificent. The defeat of Hungary at the Battle of Mohács in 1526 "sent a wave of terror over Europe."

The Muslim advance in Central Europe was halted at the Siege of Vienna in 1529, followed by a counter-attack of Charles V across the Danube river. However, by
Francis I and Charles V made peace at the Truce of Nice in 1538. Francis actually refused to meet Charles in person, and the treaty was signed in separate rooms.

Charles V, Holy Roman Emperor - Wikipedia
1541, central and southern Hungary fell under Turkish control.

Suleiman won the contest for mastery of the Mediterranean, in spite of Christian victories such as the conquest of Tunis in 1535. The regular Ottoman fleet came to dominate the Eastern Mediterranean after its victories at Preveza in 1538 and Djerba in 1560 (shortly after Charles's death), which severely decimated the Spanish marine arm. At the same time, the Muslim Barbary corsairs, acting under the general authority and supervision of the sultan, regularly devastated the Spanish and Italian coasts and crippled Spanish trade. The advance of the Ottomans in the Mediterranean and central Europe chipped at the foundations of Habsburg power and diminished Imperial prestige.

In 1536 Francis I allied France with Suleiman against Charles. While Francis was persuaded to sign a peace treaty in 1538, he again allied himself with the Ottomans in 1542 in a Franco-Ottoman alliance. In 1543 Charles allied himself with Henry VIII and forced Francis to sign the Truce of Crépy-en-Laonnois. Later, in 1547, Charles signed a humiliating[94] treaty with the Ottomans to gain himself some respite from the huge expenses of their war.[95]
Charles V made overtures to the Safavid Empire to open a second front against the Ottomans, in an attempt at creating a Habsburg-Persian alliance. Contacts were positive, but rendered difficult by enormous distances. In effect, however, the Safavids did enter in conflict with the Ottoman Empire in the Ottoman-Safavid War, forcing it to split its military resources.

Protestant Reformation
The issue of the Protestant Reformation was first brought to the imperial attention under Charles V. As Holy Roman Emperor, Charles called Martin Luther to the Diet of Worms in 1521, promising him safe conduct if he would appear. After Luther defended the Ninety-five Theses and his writings, the Emperor commented: "that monk will never make me a heretic". Charles V relied on religious unity to govern his various realms, otherwise unified only in his person, and perceived Luther's teachings as a disruptive form of heresy. He outlawed Luther and issued the Edict of Worms, declaring:

You know that I am a descendant of the Most Christian Emperors of the great German people, of the Catholic Kings of Spain, of the Archdukes of Austria, and of the Dukes of Burgundy. All of these, their whole life long, were faithful sons of the Roman Church ... After their deaths they left, by natural law and heritage, these holy catholic rites, for us to live and die by, following their example. And so until now I have lived as a true follower of these our ancestors. I am therefore resolved to maintain everything which these my forebears have established to the present.

 

Nonetheless, Charles V kept his word and left Martin Luther free to leave the city. Frederick the Wise, elector of Saxony and protector of Luther, lamented the outcome of the Diet. On the road back from Worms, Luther was kidnapped by Frederick's men and hidden in a distant castle in Wartburg. There, he began to work on his German translation of the bible. The spread of Lutheranism led to two major revolts: that of the knights in 1522–1523 and that of the peasants led by Thomas Muntzer in 1524–1525. While the pro-Imperial Swabian League, in conjunction with Protestant princes afraid of social revolts, restored order, Charles V used the instrument of pardon to maintain peace.

Thereafter, Charles V took a tolerant approach and pursued a policy of reconciliation with the Lutherans. At the 1530 Imperial Diet of Augsburg was requested by Emperor Charles V to decide on three issues: first, the defence of the Empire against the Ottoman threat; second, issues related to policy, currency and public well-being; and, third, disagreements about Christianity, in attempt to reach some compromise and a chance to deal with the German situation.[98] The Diet was inaugurated by the emperor on 20 June. It produced numerous outcomes, most notably the 1530 declaration of the Lutheran estates known as the Augsburg Confession (Confessio Augustana), a central document of Lutheranism. Luther's assistant Philip Melanchthon went even further and presented it to Charles V. The emperor strongly rejected it, and in 1531 the Schmalkaldic League was formed by Protestant princes.