Classical Age of the Ottoman Empire
Expansion continued during the first half of the reign of Suleiman I (r. 1520-1566), who conquered first Belgrade (1521) and Rhodes, before invading Hungary in 1526, defeating and killing King Louis II in the Battle of Mohács and briefly occupying Buda. Lacking a king, Hungary descended into civil war over the succession, and the Ottomans gave support to John Zápolya as a vassal prince. When their rivals the Habsburgs began to achieve the upper hand, Suleiman directly intervened by again conquering Buda and annexing it to the empire in 1541. Elsewhere, Suleiman led major campaigns against Safavid Iran, conquering Baghdad in 1534 and annexing Iraq. Ottoman rule was further extended with the incorporation of much of North Africa, the conquest of coastal Yemen in 1538, and the subsequent annexation of the interior.
Caliphate
The caliphate was claimed by the sultans of the Ottoman Empire beginning with Murad I (reigned 1362 to 1389), while recognising no authority on the part of the Abbasid caliphs of the Mamluk-ruled Cairo. Hence the seat of the caliphate moved to the Ottoman capital of Edirne. In 1453, after Mehmed the Conqueror's conquest of Constantinople, the seat of the Ottomans moved to Constantinople, present-day Istanbul. In 1517, the Ottoman sultan Selim I defeated and annexed the Mamluk Sultanate of Cairo into his empire. Through conquering and unifying Muslim lands, Selim I became the defender of the Holy Cities of Mecca and Medina, which further strengthened the Ottoman claim to the caliphate in the Muslim world. Ottomans gradually came to be viewed as the "de facto" leaders and representatives of the Islamic world. However, the earlier Ottoman caliphs did not officially bear the title of caliph in their documents of state, inscriptions, or coinage. It was only in the late eighteenth century that the claim to the caliphate was discovered by the sultans to have a practical use, since it allowed them to counter Russian claims to protect Ottoman Christians with their own claim to protect Muslims under Russian rule.