 Apart from the Turkish military and the Tahirids , al @-@ Mu 'tasim 's administration depended on the central fiscal bureaucracy . As the main source of revenue were the rich lands of southern Iraq ( the Sawad ) and neighbouring areas , the administration was staffed mostly with men drawn from these regions . The new caliphal bureaucratic class that emerged under al @-@ Mu 'tasim were thus mostly Persian or Aramean in origin , with a large proportion of newly converted Muslims and even a few Nestorian Christians , who came from landowner or merchant families . From his accession until 836 , al @-@ Mu 'tasim 's chief minister or vizier was his old personal secretary al @-@ Fadl ibn Marwan , distinguished for his caution and frugality . His replacement , Muhammad ibn al @-@ Zayyat , was of a completely different character : a rich merchant , he is described by Hugh Kennedy as " a competent financial expert but a callous and brutal man who made many enemies " , even among his fellow members of the administration . Nevertheless , and even though his political authority never exceeded the fiscal domain , he managed to maintain his office throughout the reign , and under al @-@ Mu 'tasim 's successor al @-@ Wathiq ( ruled 842 – 847 ) as well . 
