Your Question: What is the story of Imam al-Reza's succession?
The period if his Imamate was at the time of the caliphate of Harun, then ruler of Muslims, and then his sons Amin and Ma’mun. After the death of Harun, bloody wars took place between Amin and Ma’mun over the succession of their father which ended with the murder of Amin and Ma’mun became caliph as a result.
Until that days the policy of Abbasid caliphate toward the Shiites had been increasingly tough and cruel. Every once in a while one of the supporter of Ali (a.s), called “alawis”, would rebel against the caliphate so it could be really threatening for the caliph, however the Shiite Imams would not cooperate with the rebels.
The Shiite of that day continued to consider the Imams as their religious leaders and counted them as the only true caliphs of the Holy Prophet, Muhammad (pbuh). And they regarded the current caliphate as a power which is in obedience to the worldly rules rather than religious principles, like the courts of Persian kings and Roman emperors.
Ma’mun thought of finding a new solution to address the problems. He thought it could be a good choice to choose the eighth Imam as his successor in the hope that it would have different consequences, first of all it would trick the mind of the Shiite community and persuade them there was no gap between the government and the Imams and prevent them from continuing rebellions as a result and secondly, it would reduce Shiite’s inner eagerness to the Imams because being in charge in such this government would be regarded as satisfaction with what the government decided and carried out which was already considered evil and cruel and now they could remove the Imam forever.
Putting his plan into action, Ma’mun asked Imam al-Reza (a.s) to come to Marw, then capital of their administration, from Medina and sent some of his agents to Medina to force him to start the journey. He also ordered to take him through a path with the least number of Shiites. The main roads at that time were the roads to Kufa, Kermanshah, and Qom, which were all mainly Shiite cities. To prevent possible troubles, Ma’mun made Imam Reza (a.s) travel through the paths of Basreh, Ahvaz, Fars, Yazd to Marw.
Once he had arrived there, Ma’mun offered him first the caliphate but Imam al-Reza (a.s) refused so he offered him the succession to the caliphate afterwards and he refused as well but he was finally forced to accept the succession. It was just a trick because Ma’mun had set some conditions to restrict his succession to the least. One of the conditions was that Imam al-Reza (a.s) would not interfere in the affairs relating to the government and another one was that the Imam would not appoint or dismiss any of the government agents. This event took place in 200 AH (814 AD). But on contrary to what Ma’mum planned, presence of Imam al-Reza (a.s) in Iran as the successor soon led to a rapid spread of Shia, growing eagerness of the people to the Imam and an incredible warm welcome given to the Imam by ordinary people and even by the government agents. So Ma’mun realized that his evil decision just aggravated the circumstance and concluded that there was no way but having Imam al-Reza(a.s) poisoned and martyred. After the Imam died, he was buried in the city of Tus in Iran, which is now called Mashhad.
As a matter of fact, his more extensive path is :