Mehmed II (March 30, 1432 – May 3, 1481; nicknamed el-Fatih, 'the Conqueror') was the sultan of the Ottoman Empire for a short time from 1444 to 1446, and later from 1451 to 1481. He was also the first Ottoman ruler to claim the titles of caliph, supreme ruler of all Muslims, and Caesar of Rome, supreme ruler of all Christians, besides such usual titles as king, sultan (ruler of a Muslim state), Han or Khan (ruler of Turks), etc.
During his first reign, seeing the upcoming Battle of Varna, Mehmed sent for his father, Murad II, asking him to claim the throne again to fight the enemy, only to be refused. Enraged at his father, who was then retired to rest in southwestern Anatolia, Mehmed in his famous letter wrote to his father: "If you are the sultan, come and lead your armies. If I am the sultan I hereby order you to come and lead my armies." It was upon this letter that Murad II led the Ottoman army in the Battle of Varna in 1444.
Two years after reclaiming the throne in 1451, Mehmed brought an end to the Byzantine Empire by capturing Constantinople in 1453 (during the well-known Siege of Constantinople), and other Byzantine cities left in Anatolia and the Balkans. The invasion of Constantinople and successful campaigns against small kingdoms in the Balkans, Crimea, and Turkic territories in Anatolia bestowed immense glory and prestige on the country and the Ottoman State started to be recognized as an empire for the first time. Mehmed's advance toward the heart of Europe was stopped by the unsuccessful Siege of Nándorfehérvár in 1456, however.
His reign, mostly known for his capture of Constantinople, is also well known for the unusual tolerance with which he treated his subjects, especially among the conquered Byzantines. Within the vanquished city he established a milet or an autonomous settlement, and he appointed the former Patriarch as essentially governor of the city. However, his authority extended only unto the Christians of the city, and this excluded the Genoese and Venetian settlements in the suburbs, and excluded the coming Muslim and Jewish settlers entirely. This method allowed for an indirect rule of the Christian Byzantines and allowed the occupants to feel relatively autonomous even as Mehmed began the Turkish remodeling of the city, eventually turning it into the Turkish capital, which it remained until the 1920s.
As can be guessed from his successful campaign against Otranto in southern Italy and his adopting the title Roman Caesar (Kayser-i-Rüm), he was presumably trying to vitalize the Eastern Roman Empire. For a probably similar reason, he gathered Italian humanists and Greek scholars at his court, kept the Byzantine Church functioning, ordered the patriarch to translate the Christian faith into Turkish and called Gentile Bellini from Venice to paint his portrait.
He is also recognized as the first sultan to codify criminal and constitutional law long before Suleyman the Magnificent (also "the Lawmaker") and he thus established the classical image of the autocratic Ottoman sultan (padishah). After the fall of Constantinople, he founded many universities and colleges in the city, some of which are still active.