End of Empire
The last few centuries of Byzantine life were brought by a usurper, Alexius Comnenus, who began to reestablish an army on the basis of feudal grants (próniai) and made significant advances against the Seljuk Turks. His plea for western aid against the Seljuk advance brought about the First Crusade, which helped him reclaim Nicaea but soon distanced itself from imperial aid. Later crusades grew increasingly antagonistic. Although Alexius' grandson Manuel I Comnenus was a friend of the Crusaders, neither side could forget that the other had excommunicated them, and the Byzantines were very suspicious of the intentions of the Roman Catholic Crusaders who continually passed through their territory.
The Germans of the Holy Roman Empire and the Normans of Sicily and Italy continued to attack the empire in the 11th and 12th centuries. The Italian city states, who had been granted trading rights in Constantinople by Alexius, became the targets of anti-Western sentiments as the most visible example of Western "Franks" or "Latins." The Venetians were especially disliked, even though their ships were the basis of the Byzantine navy. To add to the empire's concerns, the Seljuks remained a threat, defeating Manuel at Myriokephalon in 1176.
Frederick Barbarossa attempted to conquer the empire during the Third Crusade, but it was the Fourth Crusade that had the most devastating effect on the empire. Although the intent of the crusade was to conquer Egypt, the Venetians took control of the expedition, and under their influence the crusade captured Constantinople in 1204. As a result a short-lived feudal kingdom was founded (the Latin Empire), and Byzantine power was permanently weakened.
Three Byzantine successor states were left - the Empire of Nicaea, the Empire of Trebizond, and the Despotate of Epirus. The first, controlled by the Palaeologan dynasty, managed to reclaim Constantinople in 1261 and defeat Epirus, reviving the empire but giving too much attention to Europe when the Asian provinces were the primary concern. For a while the empire survived simply because the Muslims were too divided to attack, but eventually the Ottomans overran all but a handful of port cities.
The empire appealed to the west for help, but they would only consider sending aid in return for reuniting the churches. Church unity was considered, and occasionally accomplished by law, but the Orthodox citizens would not accept Roman Catholicism. Some western mercenaries arrived to help, but many preferred to let the empire die, and did nothing as the Ottomans picked apart the remaining territories.
Constantinople was initially not considered worth the effort of conquest, but with the advent of cannons, the walls, which had been impenetrable except by the Crusaders for over 1000 years, no longer offered protection from the Ottomans. The Fall of Constantinople finally came after a two-year siege by Mehmed II on May 29, 1453. Mehmed II also conquered Mistra in 1460 and Trebizond in 1461. Mehmed styled himself the proper successor to the Eastern Roman Emperors and by the end of the century the Ottoman Empire had established its firm rule over Asia Minor and most of the Balkan peninsula.
Meanwhile, the role of the Emperor as patron of Eastern Orthodoxy had started being claimed by the Grand Dukes of Muscovy starting with Ivan III. His grandson Ivan IV would become the first Tsar of Russia. Their successors supported the idea that Moscow was the proper heir to Rome and Constantinople, a Third Rome. Both the Ottoman and the Russian Empires would continue to consider themselves proper heirs to the Byzantines until their own demises early in the 20th century.
The Byzantine empire played an important role in the transmission of classical knowledge to the Islamic world. Its most lasting influence, though, lies in its church. Early Byzantine missionary work spread Orthodox Christianity to various Slavic peoples, and it is still predominant among them and the Greeks. The start and end dates of the capital's independence, 395 to 1453, were originally the defined bounds of the Middle Ages.