In OTL, the Battle of Manzikert was a pivotal battle in 1071 between the Eastern Roman Empire and the Seljuk Turkish Empire. Due to part of the Byzantine force backstabbing the rest of the army due to political differences. The Turks, despite the fact that the Byzantines won the first day of the Battle, won the battle, and the Byzantines lost all of Anatolia in the span of a few years. The Crusades began due to this, which ultimately failed and sealed the chances for Byzantine survival.
This battle was one of the most decisive battles in history in terms of the political and cultural ramifications. It begs the question: What if Byzantium had been triumphant in the battle?
Point of Divergence1071-1200: The Byzantines as a result of a Seljuk victory, win the battle, and Anatolia remains in Byzantine hands. The Turks occasionally for the next century start insurgencies to disrupt the region. The Crusades (as we know them at last) never happen.