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Conquests in Europe

Upon succeeding his father, Suleiman began a series of military conquests, eventually leading to a revolt led by the Ottoman-appointed governor of Damascus in 1521. Suleiman soon made preparations for the conquest of Belgrade from the Kingdom of Hungary —something his great-grandfather Mehmed II had failed to achieve because of John Hunyadi 's strong defense in the region. Its capture was vital in removing the Hungarians and Croats who, following the defeats of the Albanians, Bosniaks, Bulgarians, Byzantines and the Serbs, remained the only formidable force who could block further Ottoman gains in Europe. Suleiman encircled Belgrade and began a series of heavy bombardments from an island in the Danube. Belgrade, with a garrison of only 700 men, and receiving no aid from Hungary, fell in August 1521.

The road to Hungary and Austria lay open, but Suleiman turned his attention instead to the Eastern Mediterranean island of Rhodes, the home base of the Knights Hospitaller. Suleiman built a large fortification, Marmaris Castle, that served as a base for the Ottoman Navy. Following a five-month siege, Rhodes capitulated and Suleiman allowed the Knights of Rhodes to depart. The conquest of the island cost the Ottomans 50,000 to 60,000 dead from battle and sickness (Christian claims went as high as 64,000 Ottoman battle deaths and 50,000 disease deaths).

As relations between Hungary and the Ottoman Empire deteriorated, Suleiman resumed his campaign in Central Europe, and on 29 August 1526 he defeated Louis II of Hungary (1506–1526) at the Battle of Mohács. Upon encountering the lifeless body of King Louis, Suleiman is said to have lamented: "I came indeed in arms against him; but it was not my wish that he should be thus cut off before he scarcely tasted the sweets of life and royalty." While Suleiman was campaigning in Hungary, Turkmen tribes in central Anatolia (in Cilicia) revolted under the leadership of Kalender Çelebi.

Some Hungarian nobles proposed that Ferdinand, who was the ruler of neighboring Austria and tied to Louis II's family by marriage, be King of Hungary, citing previous agreements that the Habsburgs would take the Hungarian throne if Louis died without heirs. However, other nobles turned to the nobleman John Zápolya, who was being supported by Suleiman. Under Charles V and his brother Ferdinand I, the Habsburgs reoccupied Buda and took possession of Hungary. Reacting in 1529, Suleiman marched through the valley of the Danube and regained control of Buda; in the following autumn, his forces laid the siege to Vienna. This was to be the Ottoman Empire's most ambitious expedition and the apogee of its drive to the West. With a reinforced garrison of 16,000 men, the Austrians inflicted the first defeat on Suleiman, sowing the seeds of a bitter Ottoman–Habsburg rivalry that lasted until the 20th century. His second attempt to conquer Vienna failed in 1532, as Ottoman forces were delayed by the siege of Güns and failed to reach Vienna. In both cases, the Ottoman army was plagued by bad weather, forcing them to leave behind essential siege equipment, and was hobbled by overstretched supply lines. In 1533 the Treaty of Constantinople was signed by Ferdinand I, in which he acknowledged Ottoman suzerainty and recognised Suleiman as his "father and suzerain", he also agreed to pay an annual tribute and accepted the Ottoman grand vizier as his brother and equal in rank.

By the 1540s, a renewal of the conflict in Hungary presented Suleiman with the opportunity to avenge the defeat suffered at Vienna. In 1541, the Habsburgs attempted to lay siege to Buda but were repulsed, and more Habsburg fortresses were captured by the Ottomans in two consecutive campaigns in 1541 and 1544 as a result, Ferdinand and Charles were forced to conclude a humiliating five-year treaty with Suleiman. Ferdinand renounced his claim to the Kingdom of Hungary and was forced to pay a fixed yearly sum to the Sultan for the Hungarian lands he continued to control. Of more symbolic importance, the treaty referred to Charles V not as "Emperor" but as the "King of Spain", leading Suleiman to identify as the true "Caesar".

In 1552, Suleiman's forces laid siege to Eger, located in the northern part of the Kingdom of Hungary, but the defenders led by István Dobó repelled the attacks and defended the Eger Castle.