Imam Reza (Ali ibn Musa) was the son of the seventh Imam and according to well-known accounts was born in 148/765 and died in 203/817. The eight Imam reached the imamate, after the death of his father, through Divine Command and the decree of his forefathers. The period of his imamate coincided with the caliphate of Harun and then his sons Amin and Ma'mun. After the death of his father, Ma'mun fell into conflict with his brother Amin which led to bloody wars and finally the assassination of Amin, after which Ma'mun became caliph. Until that day the policy of the Abbasid caliphate toward the Shi'ites had been increasingly harsh and cruel. Every once in a while one of the supporters of Ali (alawis) would revolt, causing blood wars and rebellions which were of great difficulty and consequence for the caliphate.
The Shi'ites Imams would not cooperate with those who carried out the these rebellions and would not interfere with their after. The Shi'ites of that day, who comprised a considerable population, continued to consider the Imams as their religious leaders to whom obedience was obligatory and believed in them as the real caliphs of the Holy Prophet. They considered the caliphate to be far from the sacred authority of their Imams, for the caliphate had come to seem more like the courts of the Persian kings and Roman emperors and was being run by a group of people more interested in worldly rule than in the strict application of religious principles. The continuation of such a situation was dangerous for the structure of the caliphate and was a serious threat to it.
Ma'mun thought of finding a new solution for these difficulties which the seventy-year old policy of his Abbasid predecessors had not been able to solve. To accomplish this end he chose the eighth Imam as his successor, hoping in this way to overcome two difficulties: first of all to prevent the descendants of the Prophet from rebelling against the government since they would be involved in the government themselves, and secondly, to cause the people to lose their spiritual belief and inner attachment to the Imams. This would be accomplished by having the Imams become engrossed in worldly matters and the politics of the caliphate itself, which had always been considered by the Shi'ites to be evil and impure. In this way their religious organization would crumble and they would no longer present any dangers to the caliphate. Obviously, after accomplishing these ends, the removal of the Imam would present no difficulties to the Abbasid.
In order have this decision put into effect, Ma'mun asked the Imam to come to Marw from Medina. Once he had arrived there, Ma'mun offered him first the caliphate and then the succession to the caliphate. The Imam made his apologies and turned down the proposal, but he was finally induced to accept the successorship, with the condition that he would not interfere in governmental affairs or in the appointment or dismissal of government agents. This event occurred in 200/814. But soon Ma'mun realized that he had committed an error, for there was a rapid spread of Shi'ism a growth in the attachment of the populace to the Imam and an astounding reception given to the Imam by the people and even by the army and government agents. Ma'mun sought to find a remedy for this difficulty and had the Imam poisoned and martyred. After his death the Imam was buried in the city of Tus in Iran, which is now called Mashhad. Ma'mun displayed great interest in having works on the intellectual sciences translated into Arabic. He organized gatherings in which scholars of different religions and sects assembled and carried out scientific and scholarly debates. The eighth Imam also participated in these assemblies and joined in the discussions with scholars of other religions. Many of these debates are recorder in the collections of Shi'ites hadiths.