In both histories, the great early empire fragments because it simply grows too big. The decaying empire is then attacked by nomadic barbarians from the north—but in the Islamic world, “the north” refers to the steppes of Central Asia and in that world the nomadic barbarians are not the Germans but the Turks. In both, the invaders dismember the big state into a patchwork of smaller kingdoms permeated throughout by a single, unifying religious orthodoxy: Catholicism in the West, Sunni Islam in the East.
