Political Motives Behind the Regency
We can summarize the causes which forced al-Mamoon to decide the issue of regency in the following:
1. In order to please the Shi'a public opinion in Khurasan and its territories which were credited with paving the road for al-Mamoon's accession to the throne and for a victory over his brother al-Amin, thus he would secure a legitimate stamp for his government when the Imam (A.S.) agreed to be his successor, since the Imam's agreement meant a recognition of the legitimacy of al-Mamoon's caliphate. Such recognition would guarantee for him the loyalty of his subjects in those regions, and I personally think that this is the most significant reason which caused al-Mamoon to do so because it would put an end to the argument of traditional opponents to the Abbaside government who used to always criticize such government and consider it illegitimate and baseless. For this reason, we can find no public discontent with the regency; on the contrary, it was a cause for elation and joyful endorsement in various circles.
It is not unlikely that al-Mamoon may have felt that some underground movement was preparing to assault his throne, snatch the government from him and hand it over to the Alawides; therefore, he tried to encircle that movement by making the Imam (A.S.) a partner with him in the forefront of the government by naming him his regent. Such an action may win him the sympathy of the Khurasanis especially after all the suffering they had to put up with and the persecution of the Abbaside caliphate which murdered them and pursued them throughout the country as fugitives in a manner which caused bitterness and agony. What supports this cause are some paragraphs of a letter al-Mamoon wrote to Banu Hashim in which he said: "You claim that I desired that they would be the recipients of benefits thereof and to be in charge, while I have in mind the interest of your posterity and children after you even while you are unaware, blindly stumbling, not knowing what plans others have in store for you."
What these ambiguous statements imply, especially after the writer admitted that the nomination of Imam Reza(A.S.) was something the man rightfully deserved due to his qualifications, is that al-Mamoon sensed the danger of the precarious political situation around him, and he feared losing his grip on the reins of government since the popular base was faithful to the Alawides. Add to this the fact that many leading elements in the political and military establishments were strong supporters of the Alawides. We can appreciate this fact by evaluating the extent of the public acceptance of the nomination of the Imam (A.S.) for the regency, and if there was any opposition, its voice was so weak it vanished amidst the tumultuous voice of overwhelming support.
Al-Mamoon did not wish the Alawides to take charge; rather, he only wished to preemptively encircle the crises which might uproot the Abbaside government if he let events shape themselves on their own.
2. To avoid a clash with the Alawides who always threatened the Abbaside government by their rebellions and uprisings during various epochs, presuming that the Abbasides had usurped the government from them, having stated that their call to uproot the Umayyads was on behalf of Reza(A.S.), descendant of the Progeny of Muhammad (S.A.W.), especially since al-Mamoon wanted his government to be stable and to avoid disturbances and crises which might weaken his position as the supreme ruler since he was still engaged in a political struggle of survival with Baghdad, the capital of the caliphate, and it was not a coincident that the issue of regency took place at the outset of that struggle.
But the Alawides had already succeeded in winning the sympathy and public support of the Islamic world and were able to maintain that to their credit. An excellent proof for that was the wide response their revolutions won among various Islamic circles. All of that was due to the violent persecution and merciless pursuits, to the murdering and banishment, and to the norms of torture and retribution from which they suffered at the hands of the ruling apparatus, so much so that even al-Mamoon testified to that in his letter to Banu Hashim quoted above.
But al-Mamoon in his afore-mentioned letter to Banu Hashim supports our argument in making this one of the causes of his decision regarding naming the Imam (A.S.) as his successor; he says, as we quoted above,"... The reason for that is my desire to safeguard your lives and protect your properties by establishing friendly ties between us and them which is a method I employ in being clement to the descendants of Abu Talib and to heel their wounds with very little of what they are entitled to."
When he ties the knot of regency for the Imam (A.S.), he wishes to put out the fire of rebellion in the souls of the Alawides and their followers and to keep the ghost of danger away should they oppose the Abbasides and try to compete with them in their bid for the government, and he did, indeed, achieve what he wanted.
3. To warn the Abbasides about what they had already done to him and how they reneged on their oath of allegiance to him, by their rebellion against him and removal from regency, that all of their actions would not disable him from overcoming them and subjecting them to his authority and, moreover, take the caliphate out of their dynasty and hand it over to their Alawide adversaries.
It is possible that the tense psychological atmosphere between al-Mamoon and the Abbasides in Baghdad posed a real challenge, and al-Mamoon found no way to force them and stir their deeply rooted sensitivities better than sending them a threatening signal that he was going to take the caliphate out of their court and throw it into that of their Alawide adversaries who constituted a point of weakness in the Abbaside psyche. Al-Mamoon found no better weapon to threaten them with stronger than that in the face of their challenges which almost uprooted his position when they all agreed to depose him in response to the call of his brother al-Amin.

Abbasides Defy al-Mamoon
The effect of that violent challenge stamped the behaviour of the Abbasides since then, for they deliberately and for the second time decided to dethrone him in a counter challenge, warning him that it would not be easy for him to get the caliphate out of their hands and hand it over to their adversaries, and that they could seat on the throne anyone they wanted from among themselves even if he had been the least qualified. In fact, they went ahead and did just that; they appointed Ibrahim ibn al-Mahdi who was nicknamed "Ibn Shakla," and who was one of the most famous singers during the entire Abbaside dynasty rule. They swore the oath of allegiance to him and others followed suit.

The Abbasides Treat Caliphate Lightly
It is very regrettable what the Abbasides did. It is an act of the most horrible nature to take so lightly the sacred institution of Islamic caliphate, and a flagrant sin committed against the most sacred divine post after Prophethood. It only shows the terrible extent of apathy reached by the Muslim masses when they accepted the nomination of such a promiscuous person as the caliph to whom they would pay homage and whom they would obey.

Al-Mamoon Backs Off and Apologizes
Such a reaction stirred the reservations of al-Mamoon as seen in a letter he wrote after the death of Imam Reza(A.S.) and during his march to Iraq. Ibn al-Athir writes: "When he (Reza(A.S.)) died, al-Mamoon wrote al-Hassan ibn Sahl informing him of Ali's death and his calamity of losing him, and he wrote to the residents of Baghdad, to the Abbasides and their subjects informing them of his death and inviting them to enter into his loyalty, and they wrote him back in the most rude manner."106
Such a violent challenge in which the Abbasides reacted to al-Mamoon and the latter's fears that they might persist in their rebellion lead the historian to conclude that al-Mamoon used one of his tricks to eliminate Imam Reza(A.S.) in order to put an end to the anger of the Abbasides and other residents of Baghdad who were outraged because of the nomination of Reza(A.S.) which was the reason why they terminated their loyalty to him.

Doubt in al-Mamoon's Sincerity
Had al-Mamoon been sincere in his intention to bring justice to the oppressed, and had he been serious in his handling of the issue of succession to the throne, why did he not name Muhammad al-Jawad (A.S.), son of Imam Reza(A.S.), after the death of his father and who was regarded as his father's successor and who was praised by al-Mamoon for his knowledge and piety and superiority to all other Hashemites? Or did al-Mamoon desire not to enter into a similar experience which might undermine his position and drag him into his downfall and collapse? Or did al-Mamoon then fulfill the purpose from which he named the Imam (A.S.) as his successor and there was no need any more to enter into another such scheme?
4. By restricting the movement of Imam Reza(A.S.) and forbidding him, after forcing him to accept regency, from demanding caliphate for himself, thus al-Mamoon secures the strangulation of the element of competition from whose nightmares his preceding caliphs used to suffer and which explained their cruel and oppressive conduct towards the Imams.

Al-Mamoon Places the Imam Under Surveillance
Al-Mamoon was not satisfied with all of that; he went ahead and subjected the Imam (A.S.) to a strict surveillance whereby he was closely watching all his internal and external movements, and he indirectly defined the extent of his contacts with others; al-Rayyan ibn al-Salt narrates the following: "Hisham ibn Ibrahim al-Rashidi was the closest person to Reza(A.S.) before he was taken to the caliph's palace, and he was a courteous and brilliant scholar. Reza(A.S.)'s contacts used to be transacted through him and under his supervision, and he used to collect all monies on his behalf before he, Abul-Hassan, was taken away. When he was taken away to the palace, Hisham ibn Ibrahim contacted Dhul-Riyasatayn and he tried his best to win his favour and started informing him and al-Mamoon about Reza(A.S.)'s movements, thus he won their confidence and did not conceal anything regarding the Imam (A.S.) from them. Al-Mamoon, therefore, appointed him as the Imam's chamberlain, and nobody could have audience with the Imam (A.S.) except whoever he liked, and he enforced a tight surveillance over the Imam (A.S.), so much so that none of his supporters could reach him without Hisham's approval, and he used to inform al-Mamoon and Dhul-Riyasatayn of anything and everything Reza(A.S.) said at home."107

Al-Mamoon's Motives Behind Enforcing Surveillance
What prompted al-Mamoon to take such a harsh measure was his great apprehension that the Alawides who predominated Khurasan, encouraged and directed by the Imam (A.S.), might move to topple his government, which is something we do not think it was logically improbable.

Strictness of Followers of the Imams
Al-Mamoon was probably aware of how followers of the Imams measured their own movements and transactions according to the prior instructions of their Imams so that their actions would be legitimate. For this reason, al-Mamoon did not need to enforce surveillance over the movements of the Alawide popular base which paid homage and allegiance to the Imam (A.S.), as much as he needed to enforce a strict surveillance over the Imam's actions and contacts. In order to verify this conduct which Shi'as have always undertaken in their practical dealings with their Imams, we ought to quote what al-Rayyan ibn al-Salt stated.
He mentioned that one Abbaside, Ibrahim ibn Hisham, used to defame and degrade the Imam (A.S.), so al-Rayyan told Reza(A.S.) about that and sought his permission to silence that man for good, whereupon the Imam (A.S.) strongly forbade him from doing so. Then he said to him, "This Fadl ibn Sahl is dispatching me to Iraq to carry out errands for him and this Abbaside man is leaving three days after that for Iraq. What do you say if I should instruct your followers in Qum to dispatch twenty or thirty men and disguise as thieves or highway robbers and when he passes by them they would attack and kill him, and people would say that he was killed by highway robbers?' The Imam (A.S.) kept silence; he neither said `Yes' nor `No;' therefore, he went to the inn-keeper and hired a horseman to go to Zakariyya ibn Adam with a letter informing him that there were matters he could not possibly include in the letter and that he would disclose them to him if he met him at such and such a place on a particular day. He said, "Leave me and the man alone," so he bade him farewell and left. The man went back to Qum where Mu'ammar had just arrived, so he consulted the matter with him, whereupon Mu'ammar said, "We do not know for sure whether his silence meant he is ordering us to do it or not. He did not explicitly order you to do anything; therefore, it is not wise to harm the man," so he changed his mind, and Zakariyya abstained from going to meet him. He passed by the Abbaside man without harming him in the least.91
Despite the fact that the Imam (A.S.) the second time did not explicitly tell al-Rayyan what to do and remained silent, having first strongly forbidden him from doing it, which gave him the impression that he approved of the plan al-Rayyan had suggested to eliminate the Abbaside man because of his silence, Zakariyya ibn Adam did not do anything except after consulting Mu'ammar in this matter who told him not to do anything since the Imam's silence could not be interpreted for sure as an order or not. This shows us the degree of precision in following the orders and instructions of the Imams.
These are the realistic reasons, within the historical understanding of the period through which the government was passing, which can be used, in part or as a whole, to realize exactly why al-Mamoon named the Imam (A.S.) as his regent.

Naive Analyses of the Regency Issues
It is naive to say that the incentive for the regency was al-Mamoon's desire to fulfill his pledge to God to hand the caliphate over to the best person among the descendants of Abu Talib if he was able to regain his post, as al-Saduq (R.A.) concluded, for such a statement, if it was indeed said by al-Mamoon in his answer to al-Fadl and his brother al-Hassan ibn Sahl, was said only to confuse the matter to others. Equally naive is that which alleged that the reason for it was to expose the reality of the Imams and their pretense of asceticism and that their pretense was only due to their inability to attain it. But al-Mamoon was more keen and more knowledgeable than others of the reality of the Imams. He knew that such posts would not in the least affect their stance and the public's regard towards them. Yet acceding to the post of caliph would not be in the eyes of the nation in conflict with the principle of asceticism if the objective is to establish an equitable society and to rule the nation by the principle of absolute justice.
The Imams and their followers, however, regard government as one of their rights which was usurped from them by others; otherwise, how can you prove that there is a conflict between one's asceticism and his acceptance of a government post? Did it undermine the asceticism of Imam Ali ibn Abu Talib (A.S.) and that of others who became caliphs and who were known for their asceticism?
These, finally, are the reasons which we can mention to clearly show us the other face of al-Mamoon revealing the real background of his politics which were ambiguous in dimension regarding his regency arrangement.

Queries
If al-Mamoon had really been serious in his offer for Imam Reza(A.S.) that he should accept the post of caliph while he himself would abdicate, or even in his offer of regency, we sill have to answer some queries without the answers to which we cannot take al-Mamoon's offer that seriously; they are the following:
1. Why did al-Mamoon send a messenger to Medina to bring the Imam (A.S.) to him escorted by a police force?
2. Why did he specify that the route he should take would pass by Basrah, al-Ahwaz, Persia, then Marw?
3. If al-Mamoon was truly convinced that the Imam (A.S.) was most qualified for caliphate, why did he not address the public on his behalf without forcing him to take such a hard journey to Marw under such specific route arrangement?
4. Why did he forbid him from leading the Eid prayers after insisting repeatedly that he should do so?
These questions may seem to some as naive and superficial, but they are deep enough to be considered in the calculation of the historian who aims at evaluating the event and its intricacies.