 In July – September 830 , al @-@ Ma 'mun , encouraged by perceived Byzantine weakness and suspicious of collusion between Emperor Theophilos and the Khurramite rebels of Babak Khorramdin , launched the first large @-@ scale invasion of Byzantine territory since the start of the Abbasid civil war , and sacked a number of Byzantine border fortresses . Following his return from Egypt , Abu Ishaq joined al @-@ Ma 'mun in his 831 campaign against the Byzantines . After rebuffing Theophilos ' offers of peace , the Abbasid army crossed the Cilician Gates and divided into three columns , with the Caliph , his son Abbas , and Abu Ishaq at their head . The Abbasids seized and destroyed several minor forts and the town of Tyana , while Abbas even won a minor skirmish against Theophilos in person , before withdrawing to Syria in September . In 832 al @-@ Ma 'mun repeated his invasion of the Byzantine borderlands , capturing the strategically important fortress of Loulon , a success that consolidated Abbasid control of both exits of the Cilician Gates . So encouraged was al @-@ Ma 'mun by this victory that he repeatedly rejected Theophilos ' ever more generous offers for peace , and publicly announced that he intended to capture Constantinople itself . Consequently Abbas was dispatched in May to convert the deserted town of Tyana into a military colony and prepare the ground for the westwards advance. al @-@ Ma 'mun followed in July , but he suddenly fell ill and died on 7 August 833 . 
