The ABBASIDES...
The ABBASIDES, who were accustomed to inflicting atrocities on the Progeny of Imam Ali (as), had learned gladiator at home. They were bound to strike each other since they never knew the qualities of compassion, sacrifice and the public good which characterized the conduct of Imam Ali descendants and for which they gave up their own rights quite often. So immediately after HARUN'S death, civil war flared up. After strife and bloodshed which continued for four years, AL-MA'MUN got the upper hand.
AL-AMIN was killed in MUHARRAM of 198 A.H. (September 813 A.D.) and AL-MA'MUN now ruled the whole ABBASIDE empire. THE POLITICAL DILEMMA After AL-AMIN'S murder, AL-MA'MUN acceded to the throne, but, as already stated above, he was half Persian on his mother's side, while AL-AMIN was of full Arab blood. AL-AMIN'S slaying had annoyed the Arab elite. Those ABBASIDE regime from time to time and were slain, imprisoned or exiled. This group, however weak, was also opposed to the ABBASIDE monarchy.
In Persia, things had taken a different turn. About seventy years prior to that era, ABU MUSLIM AL-KHURASANI had created hatred there against the UMAYYADS by telling the stories of atrocities meted by them to IMAM HUSAIN (AS) and to the rest of Imam Ali's descendants. The Persians, therefore, had compassion for the latter.
But when the tables were turned on the UMAYYADS, the ABBASIDES took advantage of the situation and seized the government, depriving the descendants of Imam Ali (as) in whose name their coup was brought about. But by and by, the Persians were gradually disillusioned. They had a full view of this political jugglery. AL-MA'MUN might have heard of the Persians' sympathy for the Prophet's Progeny.
Along with the danger of revolt of the Arab nobles, caused by AL-AMIN'S slaying, AL-MA'MUN also feared an uprising in Persia. He, therefore, considered it a political necessity to outbalance the Arabs by pleasing the Persian element and to patronize the Descendants of Imam Ali (as) by giving them a towering position in the government.
In order to conceal these political designs, he wrapped them up in the sacred cloak of "love for the Prophet's Progeny" and tried to prove that he sincerely wished to hand over the power to IMAM ALI AR-RIDA (AS). ASH-SHAIKH AS-SADUQ writes that in order to display his honesty of purpose, AL-MA'MUN propagated the story of the odds he had faced.