If this report is accurate then one has to wonder for how long Al-Sadr has understood that neither his power grab or his militia would never have been successful in Iraq:

The leader of the Shi’ite Sadrists in Iraq, Muqtada Al-Sadr, has announced in a letter to his followers that he is stepping down from his position, distancing himself from people, and focusing on his studies.

He explained that he was doing so because he had failed in carrying out his father’s will to liberate Iraq from the occupation and turning its people into believing Muslims.

He wrote that the continuation of the occupation, the many who were distancing themselves from the true path, and their turning to politics and affairs of this world had motivated him to sequester himself from human society, in order to carry out his obligation to Allah and not to add sin to the crime.

Much more: Here