His Imamate
Imam Riḍā’s (P.B.U.H.) Imamate was announced repeatedly by his father, grandfathers, and Prophet Muḥammad (P.B.U.H. & H.H.). Especially Imam Kāẓim (P.B.U.H.) mentioned him as the Imam after himself several times. For instance, one of his followers narrates, “One day Imam Kāẓim (P.B.U.H.) came to us, holding his son’s hand. We were 60 people. He said, ‘Do you know me?’ I said, ‘You are our leader.’ He said, ‘Say my name and title.’
I said, ‘You are Mūsā b. Jaʿfar b. Muḥammad.’ He said, ‘Who is this with me?’
I said, ‘ʿAlī b. Mūsā b. Jaʿfar’. He said, ‘So profess that he is my representative during my life and my successor after my death.’” He is further introduced as the eighth Imam in a saying of Prophet (P.B.U.H. & H.H.) narrated by Jābir.  Also, Imam Ṣādiq (P.B.U.H.) told Imam Kāẓim (P.B.U.H.) that the knowledge of Prophet’s (P.B.U.H. & H.H.) progeny is of your sons and he is your successor.
 
Political Situations
Imam Riḍā’s (P.B.U.H.) Imamate lasted twenty years, which can be divided into three periods:
1-The first ten years was contemporary to Hārūn’s caliphate.
2-The next five years was contemporary to Amīn’s caliphate.
3-The last five years was contemporary to Maʿmūn’s caliphate.
It was during the first period that great disasters, especially the martyrdom of Imam Kāẓim (P.B.U.H.), happened to the ʿAlavīd people (the children of Imam ʿAlī (P.B.U.H.)). Hārūn was instigated to kill Imam Riḍā (P.B.U.H.), but had not found the opportunity. After Hārūn, Amīn became the caliph. At that period, the government was weakened. Amīn was drowned in his corruption and paid no attention to Imam (P.B.U.H.) and his followers. This period was peaceful for Imam (P.B.U.H.) and his followers.
However, Maʿmūn killed his brother, Amīn, and became the caliph. He suppressed the objectors and gained the control of all the Islamic countries. He then chose Faẓl b. Sahl, who was a great politician, as his minister. But the ʿAlavīd people were a threat to his government. After suffering murder, plunder and torture for a century, they had found the opportunity to object the government and to overthrow it. They were successful in gaining people’s support, because they also had suffered great losses from the Abbasid caliphate. Therefore, Maʿmūn decided not to confront them and wanted to bring back peace and security to strengthen his government.