External world shifts left no room for a schismatic Muslim power

Evidence

The Fatimids were unfortunate in that their rule in Egypt coincided with great changes in other parts of the world—on the one side the revival of Christian power, which manifested itself in the Byzantine offensives, the reconquest of much of Spain and Sicily, and the coming of the Crusaders to the East; on the other the migration of the steppe peoples, which brought the Turks to Iraq and then to Syria, and created a new power and a new order in South West Asia. In the looming contest between Islam and Christendom, there was no room for a schismatic division on the Muslim side. — Bernard Lewis, From Babel to Dragomans, 418

Categorization

Category: Causal Relationships