 In 725 , the new Caliph , Hisham ibn Abd al @-@ Malik ( r . 724 – 743 ) replaced al @-@ Jarrah with his own brother Maslama , who already held the post of governor of the Jazira . For the time being , Maslama remained in the Jazira and was more concerned with operations against the Byzantines . In his stead , he sent al @-@ Harith ibn Amr al @-@ Ta 'i to the Caucasus front . In 725 , al @-@ Harith was engaged in consolidating Muslim authority in Albania , campaigning along the Cyrus against the regions of al @-@ Lakz and Khasmadan . He was probably also preoccupied with supervising that year 's census . In the next year , however , Barjik , one of the Khazar khagan 's sons , launched a major invasion of Albania and Adharbayjan . The Khazars even laid siege to Warthan , during which they employed mangonels . The use of such advanced siege machines shows , according to K.Y. Blankinship , that the Khazars were " a militarily sophisticated nation , not a mere disorganized pack of barbarians " . Al @-@ Harith was able to defeat them on the banks of the Araxes and drive them back north of the river , but the Arabs ' position was clearly precarious . 
