Siege of Kamacha ( 766 ) The siege of Kamacha by the Abbasid Caliphate took place in autumn 766 , and involved the siege of the strategically important Byzantine fortress of Kamacha on the eastern bank of the Euphrates river , as well as a large - scale raid across eastern Cappadocia by a part of the Abbasid invasion army . Both enterprises failed , with the siege dragging on into winter before being abandoned and the raiding force being surrounded and heavily defeated by the Byzantines . The campaign was one of the first large - scale Abbasid operations against Byzantium , and is one of the few campaigns of the Arab – Byzantine wars for which detailed information survives , although it is barely mentioned in Arabic or in Byzantine sources . Background . Following the Umayyad civil wars of the 740s and the turmoil of the Abbasid Revolution , the Byzantines under Emperor Constantine V ( reigned 741 – 775 ) regained the initiative in their eastern border and pursued an aggressive , but limited , strategy towards the Caliphate : rather than attempting a reconquest , through his deportation of frontier populations and his obstruction of Muslim fortification efforts , Constantine pursued the establishment of a permanent no - man ' s land between Byzantine and Muslim domains that would shield Asia Minor and obstruct Muslim raids against it . Among the fortresses captured by the Byzantines ( in 754 / 755 ) was Kamacha ( in Arabic : Hisn Kamkh ) . Strategically located on a plateau above the banks of the Upper Euphrates , it lay on the easternmost extremities of Byzantine territory , and since its first capture by the Arabs in 679 it had changed hands many times . After the overthrow of the Umayyads , the new Abbasid regime quickly resumed their predecessors ' attacks on the Byzantine Empire , the first being recorded in 756 . Despite a few successes on both sides , including a major Arab victory in 760 , the five years after that were relatively tranquil , with Constantine V engaged in his wars against the Bulgars and the Abbasid Caliphate focused on subduing revolts and countering Khazar incursions . Siege . In early 766 a prisoner exchange took place between the two states in western Cilicia , followed by a resumption of large - scale hostilities . In August 766 , a large Abbasid army , comprising many different national contingents , under al - Abbas ibn Muhammad , the brother of Caliph al - Mansur ( r . 754 – 775 ) , and al - Hasan ibn Qahtaba , invaded Byzantine territory from Upper Mesopotamia and made for Kamacha . The campaign is dealt with briefly by Muslim historians such as al - Tabari , but is fully recorded in a Syriac Christian source , the so - called " Zuqnin Chronicle " , written by a monk of the Zuqnin Monastery near Amida . The Abbasid force met no resistance as they pillaged their way to the fortress . Once there , they began constructing siege engines and trying to fill its moat , but their progress was obstructed by the defenders ’ own artillery . Then the Abbasids tried to launch a surprise night attack against a section of the fortress where there were no walls ; the attack was repelled by the Byzantines , who hurled great logs , weighed with stones , against them . At this point , the Muslims divided their forces : the bulk of the army , under Abbas , remained at Kamacha to continue the siege , while the remainder ( an obviously exaggerated force of 50 , 000 according to the chronicler ) was sent to raid further into Byzantine territory and pillage . The siege continued through the autumn , and the Arabs , who customarily did not take along many provisions , began suffering from lack of supplies . In the end , they established a market for merchants from Mesopotamia and elsewhere to service their needs . In the end , with winter approaching , Abbas was forced to raise the siege and retreat south , burning the large market - place to prevent it from falling into Byzantine hands . The other half of the army fared worse : lacking guides with local knowledge , it lost many men to hunger and thirst in its wanderings through the deserted borderlands , before reaching the fertile plains of Cappadocia around Caesarea . After looting the area , they turned south and made for Syria . On their way , they were encountered by a Byzantine force of 12 , 000 , which swiftly called in reinforcements . The Byzantines then attacked at night , defeating the Abbasid army and recovering its loot . The surviving Abbasid troops scattered , with some following one of their leaders , Radad , to Malatya , and some 5 , 000 under Malik ibn Tawq finding refuge in Qaliqala . It was from the latter group that the Zuqnin chronicler drew his information . The campaign is one of the few such border raids to be known in detail , and , as the Islamic historian Hugh N . Kennedy comments , " We probably get much closer to the reality of frontier warfare , with its confusions , hardships and failures , in this account than in the brief and sanitised versions provided by the Arabic historians . " Aftermath . Despite this failure , Arab pressure gradually began to mount , especially after the sack of Laodicea Combusta in 770 . The Byzantines were still capable of major counterstrokes and scored a few victories in the field , but in 782 the Caliphate mobilized its resources and launched a massive invasion under the Abbasid heir - apparent , Harun al - Rashid ( r . 786 – 809 ) , which forced the Empire into conceding a three - year truce and the payment of a heavy tribute . When warfare resumed in 785 , and until the outbreak of the Abbasid civil war following Harun ' s death in 809 , the Abbasids established and maintained a clear military supremacy , although vigorous Byzantine resistance prohibited any plans for outright conquest . Kamacha itself was surrendered to the Arabs by its Armenian garrison in 793 , only to be recovered by the Byzantines in the years following Harun ' s death . It fell again in Muslim hands in 822 , and was not finally taken by the Byzantines until 851 .