In their struggle for maintaining their supremacy in the Middle Eastern region, the Emperors of the Byzantine Empire made two mistakes:
1. They overestimated the Europeans:
In the last decade of the eleventh century, when Byzantine Emperor Alexios I Komnenos requested the Pope for military assistance, he was expecting to receive 2000 to 5000 European mercenaries who would join his army and fight the Seljuk Turks in the Middle East. What he got was a crusade in which between 50,000 to 100,000 people marched into the Middle East to wage a Holy War for liberating Jerusalem.
The crusaders were divided into factions led by preachers and warlords. They had no strategy, no knowledge of the actual conditions in the Middle East, and no unity. Many were pilgrims with no military training—they intended to disarm and defeat the enemy with their piety. Some leaders of the crusaders were looking for opportunities to found their own empires in the region. They had no intention of doing anything to help the Byzantines. In 1204, the crusaders sacked Constantinople—this was a debacle from which the Byzantine Empire never recovered.
The performance of the crusaders in the Middle East was so bad that many tribes of nomadic fighters (especially the Turks) decided to move into the fold of Islam. The Turks were led to believe that Islam, not Christianity, was the religion of victory and culture.
2. They underestimated the Seljuk Turks:
The Byzantines thought that they could deal with the Seljuk Turks in the same way that they had been dealing with other nomadic groups—that is, they could use bribes and the threat of use of military force to convert them. In the past, the Byzantines had been successful in converting the Bulgars and the Magyars to Orthodox Christianity. They failed to notice, until it was too late, that the Seljuk Turks were unlike any nomadic group that they had encountered before.
When the Seljuk Turks arrived in the Middle East, they were already a people with strong ethnic and religious identity. They had woven their Turkish identity and language into folk Islam. The Seljuk Turks were powerful fighters. Their nomadic lifestyle had prepared them for a life of warfare.
In 1055, they started raiding Anatolia. In 1071, Byzantine Emperor Romanos IV Diogenes led an army near the town of Manzikert, where he encountered the Turkish army led by Alp Arslan, the nephew of Tughril Beg, the founder of the Seljuk Empire. In the battle that followed, Romanos IV was defeated and the Turks gained control of Anatolia. In the wake of Turkish victory millions of Turks moved into Anatolia and transformed the ethnicity of this region. The Greek and Orthodox culture of Anatolia was finished by the thirteenth century.
The 1176 Battle of Myriocephalon, in which Byzantine Emperor Manuel I Komnenos was defeated, was the last Byzantine attempt to expel the Seljuk Turks from the Middle East.