Answer:
The Byzantine Empire experienced several cycles of growth and decay over the course of nearly a thousand years, including major losses during the Arab conquests of the 7th century. However, modern historians generally agree that the start of the empire's final decline began in the 11th century[citation needed].
In the 11th century the empire experienced a major catastrophe in which most of its heartland territory in Anatolia was lost to the Seljuk Turks following the Battle of Manzikert and ensuing civil war. At the same time, the empire lost its last territory in Italy to the Norman Kingdom of Sicily and faced repeated attacks on its territory in the Balkans. These events created the context for emperor Alexios I Komnenos to call to the west for help, which led to the First Crusade. However, economic concessions to the Italian Republics of Venice and Genoa weakened the empire's control over its own finances, especially from the 13th century onward, while tensions with the West led to the Sack of Constantinople by the forces of the Fourth Crusade in 1204 and the dismemberment of the empire.
Although a number of small Byzantine successor states survived and eventually reclaimed Constantinople in 1261, the empire had been severely weakened. In the longer term, the rise of Turkish power in Anatolia eventually gave rise to the Ottoman Empire which rapidly conquered the former Byzantine heartland over the course of the 14th and 15th centuries, culminating in the Fall of Constantinople to the army of Sultan Mehmed the Conqueror in 1453.
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