Battle of Rayy This Battle of Rayy ( one among many ) was fought on May 1 , 811 CE as part of an Abbasid civil war ( the " Fourth Fitna " ) between the two half - brothers , al - Amin and al - Ma ' mun Causes . Caliph Harun al - Rashid ( the father of the two brothers ) had foreseen that upon his death there would be a struggle between the two for succession to the Caliphate ( the first born succession concept used in Christian monarchies was not prevalent in Islam ) . Thus he decreed that al - Amin would rule the Abbasid Caliphate until his death , while al - Ma ' mun acted as viceroy of Khurasan province in Eastern Iran . Then al - Ma ' mun or one of his sons would succeed al - Amin as caliph , and in turn be succeeded by someone chosen by al - Amin . Naturally such a scheme , well intentioned as it was , was doomed to fail . Both brothers were influenced by their vizier s , Fadl ibn al - Rabi and Fadl ibn Sahl respectively , into attempting to claim power . Ultimately al - Amin declared that his sons would reign as Caliphs after he had died , even going as far as to seize the documents binding the brothers to the agreement from the sacred city of Mecca . Al - Ma ' mun was much angered by this and prepared for war , his vizier sending a force to defend the key frontier stronghold of Rayy , between the Zagros and Elburz mountain ranges . Meanwhile , al - Amin sent an army out to destroy his brother , which left Baghdad on March 14 , 811 CE . Leaders . Al - Amin ' s army ( divided by the numerous tribes within it ) was led by Ali ibn Isa ibn Mahan , the former governor of Khurasan , who had been ousted by Harun . Al - Ma ' mun ' s army was led by Tahir ibn Husayn , a Persian noble with some claim to one of the many mountain principalities which were nominally vassals to the caliphate . Size of Forces . Ali ' s army was " the largest and best - equipped " in living memory , numbering perhaps as many as 50 , 000 . Tahir ' s contained about 5 thousand at best , however , it contained a high proportion of cavalrymen , whereas Ali was relying mostly on infantry . Battle . Fearing what the citizens would do to rid themselves of his force if he chose to remain within Rayy ' s walls and defend against a siege , Tahir led his army along the road towards Baghdad until the intruders were in sight . After an uneasy night , Ali ' s force made an initial charge , which was followed by a brief truce and unsuccessful negotiation s , in which Tahir ' s envoy reminded Ali of the solemn agreements al - Amin had torn up . Then the main battle began , with a lightning attack force of 700 Khwarazm ians , backed by archers , sent to smash the centre of Ali ' s army , and if possible destroy its command structure . This approach was very effective , swiftly resulting in the death of Ali . How he died is not perfectly clear ; one version has it that he was a victim of an arrow - storm sent by Tahir ' s Bukharan archers ahead of the attack . Another version claims that he was felled from his horse by a soldier named Dawud Siyah ( " Black David " ) - again possibly with an arrow - but not killed . Tahir ibn al - Taji ( not related to the commander Tahir ) then approached him and asked if he was really Ali ibn Isa , following which he killed him in single combat . In either case , when he was killed his army scattered in all directions . According to Hugh N . Kennedy ; Aftermath . Tahir ibn Husayn recognised his opportunity and , without awaiting further instructions , took his almost - undamaged army on to Baghdad . After a year - long siege , Baghdad fell and al - Amin was killed .