Turks, Arabs and European crusaders
yes! its responsible for the decline of harappan civilization
wargreed bankruptcy famine one of these
No, as religion was what kept The Empire going.
The Byzantine Decline is depending upon one of there 4 events. ( I don't know which one ) * William the Conqueror converted Christianity. * The Jews were forced by the Romans to leave Judea. * Christians in western Europe organized the 1st Crusades. * A group of Muslim Seljuk Turks defeated the Byzantine army.
The Arab conquest of Byzantine Syria, Phoenicia (Lebanon), Palestine and Egypt contributed the decline of the Byzantine Empire. So did the conquest of much of what is now Turkey by the Seljuk Turks and the conquest of much of the Balkan Peninsula (southeastern Europe) by the Slavs. The Arabs and the Turks were and still are Muslims.
Yes.
The Ottoman Empire, but truthfully due to a series of civil wars and partial defeats by other empires the Byzantine empire had been in decline for centuries prior to their final defeat
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The Battle of Manzikert is seen as the beginning of the end of Byzantine control over Asia Minor. After that, the Seljuks continued to expand into Asia Minor.
The Byzantine Empire declined after Islam spread. The words "Islam spread" are nebulous. If the question is referring to the Rise of Islam period, when Mohammed and the Rightly-Guided Caliphs were spreading Islam, then yes, the Byzantine Empire decline after these events. If, however, you are referring to the Seljuk and Ottoman conquests of Anatolia nearly 600 years later which eventually ended the Byzantine Empire, the Empire was in decline long before these armies trampled the Byzantines to the ground. The answer is more complex than the above answers would purport. See the link below for another relevant answer.