Their father , Caliph Harun al-Rashid , had named Name_conferral al-Amin as the first successor , but had also named Name_conferral al-Ma'mun as the second , with Khurasan granted Giving to him as an appanage .

After Harun died Death in 809 , al-Amin succeeded Change_of_leadership him in Baghdad .

Commanded Control by the Abbasid heir-apparent , the future Harun al-Rashid , the Abbasid army reached Arriving as far as Chrysopolis , across the Bosporus from the Byzantine capital , Constantinople , while secondary forces Causation raided Attack western Asia Minor and defeated Conquering the Byzantine forces Causation there .

As Harun did not intend to assault Attack Constantinople and lacked ships to do so , he turned Becoming back .

The Byzantines , who in the meantime had neutralized Preventing_or_letting the detachment left to secure Getting the Abbasid army 's rear in Phrygia , were able to trap Conquering Harun 's army between their own converging Come_together forces Causation .

The defection of the Armenian general Tatzates , however , allowed Preventing_or_letting Harun to regain Recovering the upper hand Giving .