The Ottoman Empire began in 1299 under Osman I and soon became the most powerful Islamic empire and one of the most powerful world empires in general. This rapid expansion was due in many cases to Islam, particularly under Mehmet the Conqueror and his grandson Selim I. Even later, under Abdulhamid II, Islam remained a major unifying force. Nevertheless, Islam could not stand up to nationalism, a force that would ultimately destroy the Ottoman Empire.

Mehmet the Conqueror
Mehmet the Conqueror ascended to the Ottoman throne in 1451, and in 1453 he conquered the Byzantine city of Constantinople, which Islamic empires had tried to conquer since the religion's founding in the seventh century. To emphasize the Islamic nature of his conquest, once of Mehmet's first acts was to reconsecrate the Hagia Sophia Cathedral as a mosque.
Selim I
Mehmet's grandson, Selim I, similarly used Islam to expand the borders of the Ottoman Empire. Selim's major achievement was the conquest in 1517 of the Mamluk Sultanate, which had ruled Egypt since 1250 and whose leaders claimed the title of caliph, or spiritual leader of all Muslims. After conquering the Mamluks, Selim adopted the title of caliph, which Ottoman sultans would hold until the 20th century and which once again emphasized Islam as central to their empire.
Abdulhamid II
During the 19th century, when the Ottoman Empire was waning in power, sultans beginning with Abdulhamid II began to reemphasize the Ottoman Empire's Islamic identity. Abdulhamid not only used more Islamic calligraphy on official buildings, but also began to gather and enshrine additional Islamic relics. This re-emphasis on Islam was designed to stanch the Empire's fragmentation, as its various Islamic populations began revolting against the sultan's power.
Nationalism
Nevertheless, despite later Ottoman sultans' attempts to reassert Islam as an identity, nationalism eventually became a more powerful force than religion. The Young Turks, a nationalist and secular movement within the Ottoman Empire, overthrew Sultan Abdulhamid II in 1908. And, of course, in 1914 during World War I the British fomented a revolution in the Ottoman Empire's Arabian provinces, led by Col. T.E. Lawrence. This movement would ultimately succeed defeating the Ottomans and destroying their empire at the end of World War I.
How Muslim Empires Treated Religion
From Muhammad's founding of Islam and his unification of the Arab tribes in the seventh century, Muslims were instructed to practice respect towards other religions. This tolerance was essential to ensure peace and stability in Medina and throughout Asia Minor, as these lands were populated by Jews, Christians and other faiths. Most of the Islamic empires established in this region upheld the tradition of religious tolerance, although conflict between Sunni and Shiite Muslims was frequent.
The Seljuk Turk Empire
The Seljuk Turk Empire was established in the 10th century in the region of Anatolia, also known as Asia Minor. Within the empire, Sunni Muslims dominated and frequently clashed with the Shiite Muslim minority. In 1071, the Seljuk Turks took control of Jerusalem, and Christians in the city were persecuted for their beliefs. This led Pope Urban II to organize the first Crusade. In recapturing Jerusalem, Crusaders slaughtered members of the city's Muslim and Jewish population.
The Ottoman Empire
The Seljuk Turks were decimated by the Mongols in the 13th century. The Mongols were, however, ultimately drawn into the Islamic faith and discarded their beliefs in shamanism and Buddhism. Mongol control of Islamic lands deteriorated throughout the 14th century, eventually giving way to the rise of The Ottoman Empire. Religious tolerance was shown to Christians and Jews in The Ottoman Empire, but members of both faiths were legally prohibited from worshipping in public and were required to wear distinctive clothing.
The Safavid Empire
The Safavid Empire formed in Persia, the region that is modern-day Iran, in 1501. The empire also included parts of what are today Turkey and Georgia. Conflict with its neighboring empires was continual as the Safavid Empire was Shiite and its neighbors were Sunni Muslims. Because Shiite Islam was the state religion, Sunni Muslims fled the empire. Shiite Islam flourished, but followers of any other form of Islam -- including Sufi, the order from which the Safavids evolved -- were persecuted. All other religions were banned in the empire.
The Mughal Empire
The Mughal Empire was the last of the Islamic empires to form. It represented the union of several Indian and Pakistani kingdoms under one government during the 16th and 17th centuries. Babur, the first Mughal emperor, realized appeasement of the Hindu majority was crucial to the success of his empire and allowed Hinduism to be practiced freely. Succeeding emperors advanced religious toleration even further. Akbar the Great, Babur's grandson, had several Hindu wives, and his son is credited with establishing Urdu -- a combination of Arabic, Persian and Hindi -- as the official state language. One of the last emperors, Aurangzeb, ended religious toleration.