Sultan Mehemmed II (1451–81) had striven throughout his reign to realise one dominant aim: the consolidation of the Ottoman State. In 1453 he had conquered Constantinople which, impoverished and greatly depopulated during the last years of Greek rule, was to become once more, under his guidance, the proud capital of an empire. The sultan repaired the ancient walls and peopled the empty spaces of the city with groups of Muslims, Christians and Jews taken from all parts of his realm. Public buildings were erected, like the great mosque which he founded, with its baths and hospital, its accommodation for travellers, and its colleges where students were trained in the law of Islam. Constantinople was transformed into Istanbul, an imperial city which, reflecting in itself the rich and complex character of the Ottoman State, bound together the provinces of Anatolia and the Balkan lands as Adrianople, the former capital of the empire, could not do.

Consolidation was also the objective behind the ceaseless campaigns of the sultan. In 1459 the last remnants of independent Serbia were converted into the Ottoman frontier province of Semendria, the strong bridgehead of Belgrade, besieged in vain by Mehemmed in 1456, being left however to the Hungarians. Bosnia was overrun in 1463–4. The Bosnian aristocracy largely embraced Islam and played henceforth a great role in the defence of the frontier and in the attacks launched against Hungary and Austria. In Greece, the principality of Athens, held by a Florentine family, the Acciaiuoli, and the Greek Despotate of the Morea, ruled by the Palaeologi, were subjugated in 1458–60; while in the Aegean Sea the islands of Thasos and Samothrace, Imbros and Lemnos were taken over in 1455–6, and Lesbos in 1462.