The Tragic Ending
It was not politically feasible for al-Mamoon to reach Baghdad accompanied by Imam al-Rida (A.S.), for that would stir the winds of dissension against him and he might not be strong enough to withstand them. From this standpoint, our belief that al-Mamoon was the one who plotted to end the life of the Imam (A.S.) by giving him poisoned grapes is strengthened, and the historical environment at the time helps us confirm this belief even when Ibn al-Athir, in his Tarikh, thinks that that was not possible. Prominent scholars and historians such as Shaikh al-Mufid and others have also doubted it, while others such as Sayyid ibn Tawoos, Sibt ibn al-Jawzi, and al-Arbili in Kashf al-Ghumma, have all dismissed it outright. The latter strongly defended his view, but it was nevertheless no more than a simplistic and superficial defense. Al-Mamoon's letter to the Abbasides and the residents of Baghdad, which he wrote after the demise of Imam al-Rida (A.S.), gives such an impression. "He wrote the Abbasides and their supporters and to the people of Baghdad informing them of the death of Ali ibn Mousa and that they had resented his nomination of him as his successor, asking them now to go back to their loyalty to him.(1)
This may be understood as a clear admission that the death of the Imam (A.S.) was not natural during those circumstances, and the text Ibn Khaldun provides in expressing the contents of this letter provides even clearer clues to accusing al-Mamoon of murdering him; he says in his Tarikh:
"... And al-Mamoon sent messages to al-Hassan ibn Sahl, to the people of Baghdad, and to his supporters apologizing for naming him his regent and inviting them to go back to his loyalty." (2)
What can be understood regarding al-Mamoon's regret and realization of his mistake regarding the regency arrangement is that such regret is meaningless if it had happened after the Imam's death; rather, it must have occurred prior to that, so he paved the way to correct it by assassinating the Imam (A.S.) in order to please the Abbasides, their supporters, and the people of Baghdad. If we are to stay alone with just the political circumstances through which al-Mamoon was living during that shaky period of his reign, overlooking the historical texts whose contexts lead us to such a conclusion, we would still be able to point the finger to al-Mamoon regarding the crime of assassinating Imam al-Rida (A.S.) without being biased to any group or prejudiced against the accused.
Al-Saduq narrates saying, "While al-Rida (A.S.) was breathing his last, al-Mamoon said to him, `By God! I do not know which of the two calamities is greater: losing you and parting from you, or people's accusation that I assassinated you...'"(3)
In another narrative by Abul-Faraj al-Asbahani, al-Mamoon said to him, "It is very hard for me to live to see you die, and there was some hope hinging upon your stay, yet even harder for me than that is that people say I have made you drink poison, and God knows that I am innocent of that." (4)
This exciting situation of al-Mamoon discloses the fact that the accusation of his own murder of the Imam (A.S.) was the subject of argument, maybe even of conviction, even then, for al-Mamoon asserts people's accusation of him and he tries to extract an admission from the Imam (A.S.) clearing him of it, as Abul-Faraj mentions.
Simplistic Justification of al-Mamoon's Situation
It is interesting how some people find it hard to believe that al-Mamoon would assassinate the Imam (A.S.) simply because of all the grief, crying, abstention from eating and drinking, which he feigned to show his distress at the Imam's death, as if they expected al-Mamoon to show his happiness and excitement at his death in order to give credibility to the accusation others concealed. But the excuse of these folks is their superficiality in understanding history, and their shortsightedness.