>>648966
What destroyed the "Islamic empire" was the rise of competing dynasties like Saffarids. But it's worth debating whether it was already bound for destruction since the fitna events destroyed any semblance or hope of keeping things together for them. As if there ever was.
The shiite Fatimids were pretty isolated for instance, but they were a match for the sunni caliph, and all these different shifting dynasties can't be treated as an empire. The real danger and military threat was Isfahan and the Seljuqs which overran Asia Minor after the battle of Manzikert. They were in fact defeated by their own factions, again, and eventually the Mongols.