 In 721 / 722 , the main phase of the war began . In the winter of this year , 30 @,@ 000 Khazars launched an invasion of Armenia and inflicted a crushing defeat on the mostly Syrian army of the local governor Mi 'laq ibn Saffar al @-@ Bahrani at Marj al @-@ Hijara in February / March 722 . In response , Caliph Yazid II ( r . 720 – 724 ) sent one of his most celebrated generals , al @-@ Jarrah ibn Abdallah , with 25 @,@ 000 Syrian troops north . Al @-@ Jarrah was swiftly successful in driving the Khazars back across the Caucasus , recovered Derbent and even advanced on Balanjar . The Khazars tried to defend their capital by ringing the citadel with a laager of wagons , but the Arabs broke it apart and stormed the city on 21 August 722 or 723 . Most of Balanjar 's inhabitants were killed or enslaved , but a few managed to flee north . Despite their success , however , the Arabs had not yet defeated the main Khazar army , which like all nomad forces was not dependent on cities for supplies . The continuing threat of the Khazar army forced the Arabs to abandon any attempt to capture Samandar as well , and to retreat to Warthan south of the Caucasus . The sources are obscure on al @-@ Jarrah 's activity in 723 , but he seems to have led another campaign north ( which may indeed be the Balanjar campaign ) . In response , the Khazars raided south of the Caucasus , but in February 724 , al @-@ Jarrah inflicted a crushing defeat on them in a battle between the rivers Cyrus and Araxes that lasted for several days . Al @-@ Jarrah followed up his success by capturing Tiflis and bringing Caucasian Iberia and the lands of the Alans under Muslim suzerainty , becoming the first Muslim commander to campaign through the Darial Pass in the process . This secured the Muslims ' own flank against a possible Khazar attack through the Darial , while conversely it gave the Muslim army a second invasion route into Khazar territory . 
