On May 29, 1453, Constantinople, capital of the Eastern Roman Empire, fell to the Turks under Muhammad II; it ended the Byzantine Empire. From the article:
"Fall of Constantinople | Summary
Fall of Constantinople, (29 May 1453). After ten centuries of wars, defeats, and victories, the Byzantine Empire came to an end when Constantinople fell to the Ottoman Turks in May 1453. The city’s fall sent shock waves throughout Christendom. It is widely quoted as the event that marked the end of the European Middle Ages.
Christian Europe facing Muslim Asia. The Ottoman Turks, however, had extended their territories to include the Balkans as well as Anatolia. Only Constantinople held out behind its supposedly impregnable walls, as the Ottoman Empire spread around it. For the Ottomans, the city had enormous prestige, both as a center of the rival Christian faith and a symbol of imperial power. A attempted siege conducted by Sultan Murad II in 1422 failed, but Murad’s young successor, Mehmed II, leader of the Ottomans from 1451, was determined to carry out the operation that would cap all previous Turkish triumphs.
When Mehmed II set out to take Constantinople in the spring of 1453, the city was a shadow of its former glory, but it remained highly difficult to capture by assault. Its formidable fortifications had held out through numerous sieges in the past. It was poorly garrisoned, its defenders, under Emperor Constantine XI, numbering around 8,000 men despite having been bolstered by the arrival of Christian volunteers from across western Europe. Mehmed besieged the city in early April with a force of between 75,000 and 100,000 and a large fleet. His preparations were extensive. He had built a castle on the Bosphorus with guns that would prevent any relief ships sailing to the city from the Black Sea. He also employed a Hungarian artillery expert, Urban, to build him the most powerful cannon ever seen to batter the city’s walls. Access to the inlet of the Golden Horn, Constantinople’s port alongside the walls, was blocked by a chain, so Mehmed had his ships dragged from the Bosphorus across land on logs, then refloated in the Golden Horn to menace the fortifications from the sea.
Initial attacks on the city’s ancient but formidable walls failed with heavy casualties, but after attempts to negotiate a surrender came to nothing, the attacks began again, with increasing frequency and ferocity. Finally, on 29 May, Mehmed launched simultaneous assaults from the sea and land sides of the city that overcame the defenders. Pouring into the city through one of the gates that was forced (some claimed that it had been unlocked), the Ottomans killed the emperor as he attempted a counterattack with his remaining defenders. The Turks spread out to sack the city, massacring so many that, in the words of eyewitness Nicolo Barbaro, "blood flowed in the city like rainwater in the gutters after a sudden storm" and bodies "floated out to sea like melons along a canal." Mehmed, still only twenty years old, rode a white horse through the streets to Hagia Sophia, Constantinople’s famed cathedral, which was immediately used as a mosque to say prayers of thanks for victory.
The fall of Constantinople was a huge blow for the Christian world but, although Pope Nicholas V called for a crusade to regain the city for Christendom, no concerted military response was made. Now styled "the Conqueror," Mehmed declared the city his new capital and claimed to be the rightful successor to the Roman Empire.
Losses: Byzantine, 5,000; Ottoman, no reliable figures."