"al-Ma’mun decided to be less subtle and simply buy military force—literally, by purchasing Turkic horsemen as a slave army. Like other rulers before him, however, al-Ma’mun and his heirs learned that nomads are basically uncontrollable. By 860 the caliphs were virtually hostages of their own slave army. Without military power or religious support they could no longer generate taxes, and ended up selling off provinces to emirs: military governors who paid a lump sum, then kept whatever taxes they could extract. In 945 an emir seized Baghdad for himself and the caliphate decomposed into a dozen independent emirates.
"By then the Eastern and Western [Eurasian political-economic] cores had each fragmented into ten-plus states, yet despite the similarities between the breakdowns in the two cores, Eastern social development continued to rise faster than Western. The explanation once again seems to be that it was not emperors and intellectuals who made history but millions of lazy, greedy, and frightened people looking for easier, more profitable, and safer ways to do things. Regardless of the mayhem that rulers inflicted on them, ordinary people muddled along, making the best of things…"
--Ian Morris, Why the West Rules--for Now