Al - Abbas ibn al - Ma ' mun Al - Abbas ibn al - Ma ' mun ( died 838 CE ) was an Arab prince and general , the son of the Abbasid Caliph al - Ma ' mun ( r . 813 – 833 CE ) . A distinguished military leader in the Byzantine – Arab Wars , he was passed over in the succession in favour of his uncle al - Mu ' tasim ( r . 833 – 842 CE ) . In 838 , he was arrested for his involvement in a failed conspiracy against Mu ' tasim , and died in prison . Biography . Abbas was the only son of Ma ' mun , who in 828 – 829 appointed him as governor of Al - Jazira ( Upper Mesopotamia ) and the Mesopotamian military frontier zone ( " thughur " ) with the Byzantine Empire . Abbas distinguished himself in the expeditions against Byzantium for his bravery . In the summer of 830 , Abbas led an expedition against the Khurramite rebels of Babak Khorramdin in Azerbaijan . The campaign was accompanied by a contingent of Byzantine captives under the renegade general Manuel the Armenian , who , given Abbas ' s relative inexperience , may have been the actual commander of the army . Abbas ' s force met with some success against the Khurramites , and began its return . As it passed near the Byzantine frontier at Adata , Manuel , having earned the confidence of Abbas and his Arab officers , persuaded Abbas to cross the nearby passes and raid Byzantine territory . Once there , Manuel took advantage of a hunt to disarm Abbas and his entourage and defect back to the Empire , along with some of the other Byzantine captives . Abbas with his men were left behind and after rejoining their army , they retreated back over the mountains into the Caliphate . In the next year , however , Abbas accompanied his father and uncle in a major expedition into Byzantine Anatolia . After the Arab army crossed the Cilician Gates and took Heraclea Cybistra in early July , it divided in three corps , headed by the Caliph , Mu ' tasim and Abbas , and proceeded to raid across Cappadocia . The other two forces achieved little of consequence in the already repeatedly devastated area , but Abbas met with more success : he forced the town of Tyana to capitulate and razed it , and met and defeated the Byzantine army under the emperor Theophilos ( r . 829 – 842 ) in a minor skirmish . Ma ' mun kept up the pressure on Byzantium in 832 , with his army capturing the strategically important fortress of Loulon , and in late 832 the Caliph began gathering a huge army and announced that he intended to conquer and colonize Anatolia step by step , and finally subjugate the Empire by capturing Constantinople itself . Consequently , on 25 May 833 , Abbas with the advance force marched into Byzantine territory and began creating a military base at the site of Tyana . The site had been fortified and awaited the arrival of the Caliph ' s army , which in early July crossed into Anatolia . At his juncture , however , luck intervened for the Byzantines , as Ma ' mun fell ill and died , although some modern scholars speculate that his death may have been the result of a coup . Although Abbas was Ma ' mun ' s son , the Caliph apparently had named his brother Mu ' tasim as his heir shortly before his death . Abbas swiftly swore allegiance to Mu ' tasim , but this turn of events was not popular among the assembled army , which tried to proclaim Abbas caliph . Abbas refused , and managed to assuage the troops ' anger . Nevertheless , Mu ' tasim ' s hold on the throne was still shaky , and he abandoned Ma ' mun ' s campaign ; the new base at Tyana was razed , and the army returned to the Caliphate . Despite his acceptance of his uncle ' s succession , Abbas became the focus of the factions opposed to Mu ' tasim , and in particular his increasing reliance on and favour shown to his Turkish slave - soldiers ( " ghilman " ) . This discontent resulted in a conspiracy led by the general ' Ujayf ibn ' Anbasa which aimed to kill Mu ' tasim and place Abbas , who was aware of the plot , on the throne . The conspiracy was uncovered while Mu ' tasim was campaigning against the Byzantines , with the Caliph informed of it just after his army had sacked Amorium . The resulting investigation , headed by Mu ' tasim ' s trusted Turkish general Ashinas , resulted in the execution of most conspirators . This was broadened into a virtual purge of the army , in which the hitherto dominant Khurasani element was replaced with Mu ' tasim ' s favoured Turks . Abbas himself was imprisoned and died in prison at Manbij in 838 , while his male descendants were imprisoned and executed by Ashinas .