Byzantine Defeat at Manzikert
In the east of Europe was the Byzantine Empire, Christians who had long followed a separate Orthodox rite.
Since 1054 the Eastern Orthodox and Roman Catholic churches had been in schism, and it has been argued that the desire to impose Roman church authority in the east may have been one of the goals of the crusade, although it should be noted that Urban II, who actually launched the First Crusade, never refers to such a goal in his letters on crusading. The Seljuk Turks had taken over almost all of Anatolia after the Byzantine defeat at Manzikert in 1071, with the result that on the eve of the Council of Clermont, the total territory controlled by the Byzantine Empire had fallen by more than half. By the time of Emperor Alexius I Comnenus, the Byzantine Empire was largely confined to Balkan Europe and the northwestern fringe of Anatolia, and faced Norman enemies in the west as well as Turks in the east. In response to the defeat of Manzikert and subsequent Byzantine losses in Anatolia in 1074, Pope Gregory VII had called for the milites Christi ("soldiers of Christ") to go to Byzantium's aid. This call, while largely ignored and even opposed, nevertheless focused a great deal of attention on the east.
The Battle of Manzikert, or Malazgirt, was fought between the Byzantine Empire and Seljuq forces led by Alp Arslan on August 26, 1071 near Manzikert (modern Malazgirt in Muş Province, Turkey). It resulted in one of the most decisive defeats of the Byzantine Empire and the capture of the Byzantine Emperor Romanos IV Diogenes. The Battle of Manzikert played an important role in breaking the Byzantine resistance and preparing the way for Turkish settlement in Anatolia.
The battle marked the high point of the initial Turkish incursions and was followed up two years later with a large influx of Turkish settlers and soldiers, many at the request of the crumbling Byzantine Empire. However, the battle was not the slaughter that many historians, including contemporary writers, have stressed it to be — large numbers of mercenaries and Anatolian levies fled and survived the battle, thanks in part to Alp Arslan's refusal to pursue them. All the Byzantine commanders, including Romanos, survived to participate in the numerous civil conflicts that wrecked Anatolia. Nonetheless, the Byzantine Empire would never again be able to muster a force as large, nor as distantly projected, as that which took part in the fateful battle.
Wikipedia: Battle Of Manzikert
Wikipedia: First Crusade