Harthama ibn A'yan had been in Samarkand when al-Rashid died at Tus in March 809, and remained in the east after.

Consequently he had thrown in his lot with al-Ma'mun in the civil war against al-Amin, and is along with Tahir ibn Husayn one of the two commanders of al-Ma'mun's army during the crucial year-long siege of Baghdad.

During the siege Harthama leads the attack from the east while Tahir commands from the west.

In the final stage of the siege, Harthama tries unsuccessfully to secure the surrender and life of al-Amin, who has in the meantime been declared deposed as caliph in Iraq and Arabia, by sending a boat to ferry him over the Tigris.

The boat, however, capsizes, and al-Amin is captured and executed by Tahir's men, contrary, it seems, to al-Ma'mun's orders, on September 24 or 25, 813.

The father of Hassan ibn Sahl was an Iranian Zoroastrian convert to Islam.

Along with his brother, the future vizier al-Fadl ibn Sahl, Hasan had entered the service of the Barmakid al-Fadl ibn Yahya in the reign of Harun.

During the civil war, he has been entrusted with the supervision of the land tax (kharaj) office.

After Ma'mun's troops capture the caliphal capital, Baghdad, Hasan is sent west to assume the governance of Iraq, while Ma'mun and Fadl remain in Marv.

Al-Ma'mun rules from 813 as the recognized caliph, but Egypt, Syria, and Mesopotamia begin to slip from central control.

The appointment of 'Ali ar-Rida arouses varying reactions—few of them, even among the Shi'ites, wholly favorable—and Iraq, already irritated by al-Ma'mun's transfer of the capital from Baghdad to Merv and by other offenses, rises up in rebellion.

In July 817, after news reaches the city that Ma'mun has chosen an Alid, Ali ibn Musa al-Rida, as his heir-apparent, the city appoints his uncle, Ibrahim, son of the third caliph, al-Mahdi, as Caliph.

The vizier had been assassinated in February 818 during al-Fadl’s long journey from Merv to Baghdad, and 'Ali ar-Rida dies in August 818, after a brief illness, at Tus. (Shi'ite historians attribute his death to poison, possibly administered by the caliph himself. His shrine (mashhad) at Tus becomes a pilgrimage place and gives its name to the city Mashhad, or Meshed. The Shi'ites attribute many miracles to 'Ali ar-Rida.)

Fadl ibn al-Rabi had reemerged briefly from hiding during this time in support of Ibrahim, but when Ma'mun enters the capital in 819, he secures his pardon.

During his last years, Fadl will even enjoy a return to the Caliph's favor due to his long experience and loyal service to the Abbasid house.

He will die in Baghdad in the spring of 823 or 824.

Following Fadl' ibn Sahl’s murder in 818 and Ma'mun's entry in Baghdad in 819, there are expectations that Hasan will succeed his brother as vizier of the Caliphate, but Hasan, shocked by Fadl's death, prefers to retire from politics and withdraw to his estates around Wasit, where he will remain until his death in 850/1.

In 825, however, he will marry his daughter, Buran (807–884), to Ma'mun, and give her as dowry the palace of Qasr al-Hasani, south of Baghdad, which thereafter will become one of the caliphal residences.

Caliph al-Ma'mun’s return to Baghdad secures the city's place as the capital of the Abbasid Caliphate.