 Whatever the true impact of Marwan 's campaigns , warfare between the Khazars and the Arabs ceased for more than two decades after 737 . Arab military activity in the Caucasus continued until 741 , with Marwan launching repeated expeditions against the various princes of the northern Caucasus , most notably Tuman Shah . Nevertheless , these campaigns , according to Blankinship , seem to have been closer to raids , designed to seize plunder and extract tribute to pay for the upkeep of the Arab army , rather than attempts at permanent conquest . Despite the Umayyad success at establishing a more or less stable frontier anchored at Derbent , Blankinship is also critical of the long @-@ term results of the Second Arab – Khazar War : Arab control was in reality limited to the lowlands and coast , and the land itself was too poor to recompense the expenses sustained during the wars . Furthermore , the need to maintain the large garrison at Derbent further depleted the already overstretched Syro @-@ Jaziran army , the main pillar of the Umayyad regime . Eventually , this weakening of the Syrian army would be the major factor in the fall of the Umayyad dynasty during the Muslim civil wars of the 740s and the Abbasid Revolution that followed them . 
