Selim’s last years were spent in Istanbul solidifying the supremacy of the sultanate, exploiting the prestige and revenues that resulted from his Eastern victories. It was therefore only during the long reign of his son and successor, Süleyman I (ruled 1520–66), called “the Magnificent” in Europe and “the Lawgiver” (Kanuni) among the Ottomans, that the foundations laid by Selim were fully used to establish the classical Ottoman state and society and to make important new conquests in the East and West. As a result of his father’s policies and successes, Süleyman assumed the throne with a position unequaled by any sultan before or after. He was left without opposition and with a great deal of control over the devşirme class, as well as over the remnants of the Turkish notables. The conquest of the Arab world had doubled the revenues of the treasury without imposing important additional financial obligations, leaving Süleyman with wealth and power unparalleled in Ottoman history. Although Süleyman never took full advantage of the opportunities left him and, in fact, began the process of Ottoman decline, his reign still marked the peak of Ottoman grandeur and has always been regarded as the golden age of Ottoman history.

The chief battlefields of Ottoman expansion in Europe under Süleyman were Hungary and the Mediterranean. The weak southeastern European enemies of Süleyman’s predecessors had been replaced by the powerful Habsburg dynasty, which was bolstered by the appeals of the pope throughout Europe against the menace (to Christians) of Islam. Süleyman’s main European ally was France, which sought to use Ottoman pressure in the south to lessen the pressure of the Habsburgs on its eastern frontiers. The land war with the Habsburgs was centred in Hungary and was fought in three main stages. From 1520 to 1526 the independent Hungarian kingdom bore the direct brunt of the Ottoman attack and acted as a buffer between the two great empires, but the weak king Louis II of Hungary and Bohemia and feudal anarchy and misrule made a united defense impossible. A split among Hungarian nobles over the question of accepting Habsburg rule, combined with social and national divisions stimulated by the Reformation, further weakened the opposition to Ottoman attack. As a result, Süleyman was able to take Belgrade in 1521, opening the way for a large-scale advance north of the Danube. The only real army the Hungarian nobles could muster was routed in 1526 at the Battle of Mohács, and the death of Louis II ended the last hope for Hungarian unity and independence.

The second period of Ottoman-Habsburg relations (1526–41) was characterized by Hungarian autonomy under the anti-Habsburg Hungarian king John (János Zápolya), who accepted the suzerainty of the sultan in return for the right to continue native administration and military defense. The Habsburg prince Ferdinand (later the Holy Roman emperor Ferdinand I), brother of the emperor Charles V, occupied the northern areas of Hungary with the support of the wealthier Hungarian nobles who desired Habsburg aid against the Turks. For all practical purposes he annexed them to Austria before undertaking to conquer the remainder of Hungary in 1527–28. In response Süleyman returned from Anatolia to drive the Habsburgs from all of Hungary and besieged Vienna in 1529, an effort that failed because of the difficulty of supplying a large force so far from the major centres of Ottoman power.

Vienna thus stood as the principal European bulwark against further Muslim advance. Under the existing conditions of supply, transport, and military organization, the Ottomans had reached the limit of their possible expansion in the West; the winter base that supported the expansion effort had to be maintained in Istanbul because of the constant threat of military action against the Safavids in the East. The siege of Vienna, however, secured Süleyman’s rule of Hungary and prevented Ferdinand from launching a new attack against the territories ruled by John until 1540. Although the siege frightened the other states of Europe sufficiently for them to agree to a truce between Roman Catholics and Protestants (1532), the result was only temporary, and Ferdinand never was certain of the support of the independent German princes and the other European rulers who promised help. Even Charles V was too preoccupied with the problems of the Reformation and with France to devote much attention to the Ottomans. Thus, when Süleyman embarked on a second Austrian campaign (1532), he was unable to draw the imperial army into conflict and had to content himself with devastating large areas of the Habsburg realm.

By the peace of 1533, Ferdinand abandoned his claims to central Hungary and recognized John’s rule there as Ottoman vassal, while Süleyman agreed to accept Ferdinand as ruler of northern Hungary in return for the payment of an annual tribute. That arrangement lasted until 1540, when John died and left his dominions to Ferdinand in defiance of his agreement with the sultan. When Ferdinand tried to assume his heritage by force, Süleyman occupied and annexed Hungary in 1541—under the guise of championing the cause of John’s infant son, John Sigismund Zápolya—putting it under direct Ottoman administration and occupation for the first time. Thus began the third and final period of Ottoman-Habsburg relations, characterized by continuous border conflict; diversions on both sides, however, prevented long periods of open warfare. Christian historians have accused Francis I of France of encouraging Ottoman expansion into central Europe to relieve Habsburg pressure on him. But the Ottoman advances should be ascribed less to French overtures than to Süleyman’s own ambitions, together with his fears of Habsburg rule in Hungary and a possible alliance among the Habsburgs, Hungarians, and Safavids.

The sultan regarded the French king largely as a supplicant for commercial favours, which were granted in the Capitulations treaty of 1536, an agreement by which French subjects were given the freedom to travel and trade in the sultan’s dominions and subjects of other states wishing to do the same were required to secure French protection. French and other merchants and travelers in the Ottoman Empire were allowed to remain under French laws and courts in cases concerning themselves and were granted special privileges in cases involving Ottoman subjects. Thus was established the foundation of the French predominance in the Levant (region along the eastern Mediterranean), which remained until modern times. The Capitulations served as a model for later agreements between the Ottomans and the other European powers, who subsequently used them during the centuries of Ottoman weakness as means to dominate commerce within the Ottoman dominions and to drive the native Muslims and Jews out of the marketplace in favour of their coreligionist Greek and Armenian protégés. The stalemate between the Ottomans and Habsburgs in northern Hungary was characterized by centuries-long conflicts along the land frontier. Periodic Ottoman raids into central Europe and resulting European anti-Muslim propaganda led to Christian prejudice against Muslims in general and Turks in particular; many Europeans sympathized with the Christian minority subjects of the Turks, a sentiment that lasted into modern times.

Organized military conflict shifted to the sea, with the Ottomans emerging for the first time as a major naval power. The decline of the Venetian navy led Charles V to seek complete control of the Mediterranean, enlisting as his naval commander the great Genoese seaman Andrea Doria and thus gaining the support of the powerful Genoese fleet. Süleyman responded in 1522 by driving the Knights of Rhodes, a Christian religious and military order, out of Rhodes, but in 1530 Charles established them on Malta, from which they organized piratical raids against Ottoman ships and shores and in 1535 captured Tunis. While Süleyman was occupied in Anatolia, Doria captured a number of ports in the Morea and began to raid the Ottoman coasts, severing most sea lines of communication between Istanbul and Alexandria and preventing thousands of Muslim pilgrims from reaching Mecca and Medina. In response, Süleyman in 1533 enrolled in his service as grand admiral Khayr al-Dīn (known to Europeans as Barbarossa), a Turkish captain who had built a major pirate fleet of “sea ghazis” in the western Mediterranean and used it to capture Algiers (1529) and other North African ports. As part of the arrangement with Barbarossa, the Ottomans annexed Algiers to the empire as a special timar province permanently assigned to the grand admiral to support the fleet. Ottoman land troops were sent to defend Algiers against Habsburg attacks, which probably was the main reason Barbarossa agreed to join the sultan. Barbarossa built a powerful Ottoman fleet able to confront the Habsburgs on equal terms. In 1537 he launched a major attack on southern Italy, expecting a promised French attack in the north, with the objective of a joint conquest of Italy. But France, fearing a hostile European reaction to its alliance with the infidel, withheld the diversion. Doria then organized and led an allied European naval force against the Ottomans, but it was routed in 1538 at the Battle of Préveza off the Albanian coast. Venice then surrendered the Morea and Dalmatia, its last possessions in the Aegean Sea, thus assuring an Ottoman naval supremacy in the eastern Mediterranean that remained unbroken for three decades.

Süleyman failed to pursue his ambitions in Europe after 1541, largely because of his increasing preoccupation with problems in the East. He ruthlessly suppressed Safavid propagandists and supporters in eastern Anatolia and stimulated the Uzbek empire of Transoxania to attack Iran. Iran fell into disorder following the death of Ismāʿīl and the accession of his infant son Ṭahmāsp I, but Süleyman was able to take advantage of that situation only during periods of peace in Europe. He personally led three campaigns into northwestern Iran, in 1534–35, 1548–50, and 1554, and, although he captured Safavid territories in the southern Caucasus range and in Iraq, he never was able to catch and defeat the Iranian army. Supply problems invariably compelled him to retire to Anatolia during the winter months, allowing the Persians to regain Azerbaijan with little difficulty. Süleyman finally despaired of defeating his elusive enemies and agreed in 1555 to the Peace of Amasya, by which he retained Iraq and eastern Anatolia but renounced Ottoman claims to Azerbaijan and the Caucasus and agreed to allow Shiʿi Persian pilgrims to visit Mecca and Medina as well as their own holy places in Iraq. Thus, the same geographic problems that had limited Ottoman conquests in central Europe made western Azerbaijan the practical limit of Ottoman expansion in the East, preventing the final elimination of the Safavid danger.

Süleyman was somewhat more successful in restoring the old international trade routes through his Middle Eastern possessions. To counteract the Portuguese fleet, supplied by the Safavids from their Persian Gulf ports, he built major naval bases at Suez (1517) and, as soon as he took Iraq, at Basra (1538), establishing garrisons and fleets that not only resisted the Portuguese naval attacks but also attacked them in the Eastern seas. As a result, the old trade route regained some of its former volume in the 16th century. The Ottomans never were able to fully restore it, however, because Portugal, using a sea route, was still able to pay higher prices in the East and sell at lower prices in Europe, avoiding the duties and local charges levied on goods sent by land through Ottoman territory. It should be noted that, contrary to the myths maintained by many European historians, it was the Ottomans who fought to keep the old Middle Eastern trade route open; the route was closed only when the Cape route was taken over from the Portuguese by the much more powerful fleets of the English and Dutch.