The causes of the First Crusade are widely debated among historians. By the start of the 11th century Europe, the influence of the papacy had declined to that of little more than a localised bishopric.
Compared with Western Europe, the Byzantine Empire and the Islamic world were historic centres of wealth, culture and military power. The first waves of Turkic migration into the Middle East enjoined Arab and Turkic history from the 9th century. The status quo in Western Asia was challenged by later waves of Turkish migration, particularly the arrival of the Seljuk Turks in the 10th century.