Perhaps because of his final failure in Khurasan and his loss of the province to the Saffarids, Muhammad b. Tahir is viewed in the sources as a markedly inferior figure compared with his predecessors, and as a weak and neglectful voluptuary. He was unfortunate in that, soon after his assumption of power, the Caspian provinces broke out in a general revolt so serious and lasting in its effects that outside control could never be fully re-imposed there. This Zaidi Shi'i revolutionary movement in Tabaristan is, in fact, an early manifestation of the rise of hitherto submerged northern Iranian elements, above all of the Dailamites which was to characterize the 4th/10th and early 5th/11th centuries. The middle decades of the 3rd/9th century also witnessed widespread activity by various 'Alid claimants, who seized their opportunity as the 'Abbasid caliphate became paralysed at the centre with uprisings and military coups. In 250/864 there occurred the revolt of Yahya b. 'Umar in the region of Kufa in Iraq, which was suppressed by Muhammad b. c Abd-Allah b. Tahir's forces. A much more serious threat was posed by the movement of al-Hasan b. Zaid in Tabaristan. Maladministration by members of the Tahirid family and their officials contributed much to popular exasperation there, leading in the end to revolt. 'Iraq 'Ajami or western Persia, including the Caspian provinces, came under Muhammad b. 'Abd-Allah, who had appointed his brother Sulaiman as his deputy in Tabaristan and Gurgan. Sulaiman's officials, and especially one Muhammad b. Aus al-Balkhi, behaved oppressively. Particularly resented were the actions of a Christian official of the Tahirids on some caliphal estates at Chaliis and Kalar, on the borders of Tabaristan and Dailam, which had been granted to a certain Muhammad b. 'Ab-Allah. This agent had confiscated mawt "dead" lands (i.e. uncultivated ones) formerly used by the local people as common pasture. There followed a rising of the people of Tabaristan and Ruyan, headed by two "sons of Rustam", and aided by the Dailamites of the mountains to the west. The 'Alid al-Hasan b. Zaid, called al-Da'i al-Kabir "the great summoner to the true faith", then came from Ray. He was generally recognized as amir of Tabaristan; Sulaiman b. 'Abd-Allah and the Tahirid tax-collectors were expelled to Gurgan, and for a time, the insurgents even held Ray. Sulaiman's ignominious defeat did not prevent his nomination to the governorship of Baghdad and the Sawad in 255/869, two years after his brother Muhammad had died in these offices; it did, however, bring down on his head the satires of the poet Ibn al-Rumi.