In the struggle for control of the Levant, the Byzantines made two fatal 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 Asia Minor. What he got was a crusade in which between 50,000 to 100,000 people marched into the Levant proclaiming that they intended to wage a Holy War for liberating Jerusalem. They failed to realize that the call for Holy War from one side would inspire a call for Holy War from the other side.
The crusaders were divided into factions led by preachers and warlords. They had no strategy, no knowledge of the actual conditions in the Levant, 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 Levant, and 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 Levant was so bad that many tribes of nomadic fighters (including some factions of Turks) who had been attracted to Orthodox or Latin Christianity decided to move into the fold of Islam, because they thought that Islam was the religion of victory and culture.
Even if Europe had sent its best troops to the Levant, they could not have defeated the Islamic forces. From the fall of the Roman Empire to the twenty-first century, the European powers have not won a single military victory in a war outside Europe against a powerful enemy. They have won victories against primitive and weak communities in the Americas and other places during the age of imperialism but whenever they encounter a powerful enemy outside Europe, who is determined to fight, they fail to make a headway in the battlefield.
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 were 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 Asia Minor, 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 to fight brutally to win decisive victories.
In 1055, they started raiding Anatolia. In 1071, Byzantine Emperor Romanos IV Diogenes led a large 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, which has been a Turkish nation ever since. In the wake of Turkish victory millions of Turks moved into Asia Minor and transformed the ethnicity of this region. The Greek and Orthodox culture of Asia Minor 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 Asia Minor.