 On his deathbed , al @-@ Ma 'mun dictated a letter nominating his brother , rather than his son al @-@ Abbas , as his successor . This appointment owed much to Abu Ishaq 's strong personality and leadership skills , but also because he was the only Abbasid prince to control independent military power in the form of his Turkish corps , on which al @-@ Ma 'mun had come to increasingly depend . Consequently , Abu Ishaq was acclaimed as Caliph on 9 August , with the name of al @-@ Mu 'tasim . His position was far from secure , however , as a large part of the army favoured al @-@ Ma 'mun 's son Abbas , and even tried to proclaim him as the new Caliph . Consequently al @-@ Mu 'tasim called off the expedition , abandoned the Tyana project and returned with his army to Baghdad , which he reached on 20 September . 
