We read in history books that Ma’mun, the Abbasid caliph, offered caliphate to Imam Reza (PBUH), but he did not accept it. Then, Ma’mun offered the Imam (PBUH) to become his successor, but Imam Reza (PBUH) again refused; so Ma’mun threatened to kill the Imam (PBUH), and since Imam Reza (PBUH) found this threat to be serious, he was forced to accept Ma’mun’s succession.
In fact, Ma’mun was hoping to establish a powerful and nationwide base for his kingdom. He thought that since Imam Reza (PBUH) had a powerful reputation, he would gain the same reputation for his kingdom by appointing the Imam (PBUH) as his successor. For example, when he was asked by Hamid ibn Mihran and some other Abbasids about the reason for this appointment,
he replied by saying,” This man (Imam Reza (PBUH)) was out of our sight. He used to attract people towards himself. Thus, we made him become our successor so that attracting people towards himself would benefit us. Also, he would approve and submit to our kingdom, and his lovers would find his claims void. We feared that he would make the situation intolerable for us if we let him free...”
Imam Reza (PBUH) accepted to become Ma’mun’s successor in a situation that he had to save the lives of himself and his Shiites from dangers. He had to remain alive so that he would be the guide, leader, and reference of people in their difficulties and questions. As such, Imam Reza (PBUH) took advantage of this opportunity and performed his responsibility of enlightening people.
He established an extensive network through people, his writings were spread all to way to the east and west borders of the Islamic nation, and all different groups of people were attracted to him. Besides that, during this period, Imam Reza (PBUH) revealed the true character of Ma’mun to people, and he cleared any doubts or misunderstandings by disclosing the intentions of Ma’mun behind his actions.
Nevertheless, the virtues of Imam Reza (PBUH) were so obvious that even Ma’mun, who was the Imam’s (PBUH) number one enemy, has admitted to them in numerous occasions. On year 200, when Ma’mun had gathered over thirty three thousand Abbasids, he addressed them by saying,” ... I searched amongst the children of Abbas and Ali so much, and I found none of them more virtuous, pious, religious, worthy, and suitable for my succession than Ali ibn Musa al-Reza (1)”. Moreover, in a writing to the Abbasids, he described the Imam (PBUH) as “... As for paying allegiance to Ali ibn Musa,
I have asked you to do so after finding him worthy of this position, and I have chosen him myself... And about the question you asked whether I have had enough insight for this appointment, you should know that I did not pay allegiance to him but for I have realized that there is no one on the earth that is more virtuous and pious than him, or superior to him in terms of asceticism and freedom. No one satisfies people better than him, and no one is more firm in the way of Allah (SWT) than him...(2)”.
It is clear that Ma’mun was not truthful in his confessions, and he was only admitting to them to pursue his evil intentions. However, by reminding these statements, we realize the status and character of Imam Reza (PBUH). Is it not said that,” Virtues are those which the enemies testify”?
(Selection taken from “The Political Life of Imam Reza (PBUH)”, by Sayyid Ja’far Murtada al-Aamili, translated (to Persian) by Sayyid Khalil Khalilian (with some changes))
Roshd website offers its condolences upon the last day of Safar, the anniversary of the martyrdom of the helper of the poor, Imam Ali ibn Musa al-Reza (PBUH), to all Muslims around the world, and especially to you dear friend.

Footnotes:
1. Morouj al-Dhahab, vol. 3, p. 441- Al-Kamil by Ibn Athir, vol. 5, p. 183- Al-Aadab al-Sultanyyah, p. 217- Tarikh Tabari, vol. 11, p. 1013 (Published in London) -Mukhtasar Taarikh al-Dowal, p. 143- Tajaarib al-Umam, vol. 6, p. 436
2. Nadharyyah al-Imamyyah, p. 388