Islam has, throughout the history of its aggression against the West, benefited immensely by, and in many cases cunningly exploited the divisions within the West. Some of the first Byzantine provinces to fall after the Mohammedan Revolution were those, like Egypt and North Africa, whose internal repose had already been shattered by the conflagrations of the great heresies of Christian antiquity. Many Arian, Nestorian, and Donatist communities had been subject to oppressions and persecutions from the Empire in the decades immediately preceding the rise of Islam, and they welcomed the Muslim invader, even, in some cases, collaborated with him. The rabble of the People’s Crusade ravaged the Anatolian countryside — populated, by and large, by Greek Christians — before succumbing to the Turkish counterattack. The Crusader Bohemund, who betrayed his oath of fealty to the Emperor, made himself Prince of Antioch and never marched with the rest to Jerusalem, later returned to Rome and convinced the Pope that the real enemies of the Latin West were the Byzantines. His intrigues in the Vatican were as damaging to the endurance of Christendom as anything the Turks ever did.
When Constantinople was besieged in 1453, there were many Greek Christians among the Sultan’s legions (and some few Turks among the city’s defenders). A Christian engineer oversaw the construction of the guns that broke the city’s great walls. The Venetians squabbled with the Genoese, and only sent a relief convoy after it was too late (though many individual Venetians and Genoese fought valiantly to the end). It is estimated that when she finally fell to Mehmet II, the great capital of Eastern Christianity, which had stood for eleven and a half centuries, could summon only a mere four thousand men to defend her.Even at our great victory at Lepanto, the unity of the Christian forces was achieved only by extraordinary efforts, and by the extraordinary leadership of Don John of Austria.
You can read it all at Enchiridion Militis.