Hasan ibn Zayd

Abū Muḥammad al-Ḥasan ibn Zayd ibn Muḥammad ibn Ismaʿīl ibn al-Ḥasan ibn Zayd (; died 6 January 884), also known as al-Dāʿī al-Kabīr (, "the Great/Elder Missionary"), was an Alid who became the founder of the Zaydid dynasty of Tabaristan. In 864, he was living in Ray in northern Iran, when he was invited by pro-Alid elements in the neighbouring province of Tabaristan to join them in a revolt against the Abbasid authorities. It was only after 840, when Tabaristan came under Tahirid rule (the Abbasids' viceroys for the East), that the Islamization of the province began. It proceeded rapidly, and although the majority of the people adopted Sunni Islam, the province offered opportunities for the activities of pro-Alid Shi'ite missionaries as well.

In the 860s, western Iran was governed by the Tahirid Muhammad ibn Abdallah ibn Tahir, whose brother Sulayman ibn Abdallah ibn Tahir deputized him in Tabaristan and Gurgan. Popular resentment of the Tahirids' rule increased through the oppression of their officials, especially their fiscal agents in the province. Consequently, in 864 a rebellion broke out in the towns of Ruyan, Kalar and Chalus, led by two "sons of Rustam". The rebels called upon Hasan to lead them, and allied themselves with the neighbouring Daylamites. Hasan, who assumed the regnal name al-Dāʿī ilaʾl-Ḥaqq ("He who summons to the Truth"), was recognized as emir by a part of the local population, and even secured the allegiance of the Justanid king of Daylam, Vahsudan ibn Marzuban. Thus he was chased out of Tabaristan in 865 by Sulayman ibn Abdallah, but returned within the same year and recovered the province. Hasan was forced into Daylam again by the Abbasid general Muflih in 869, but the latter withdrew shortly after. In the complex struggle for control of Khurasan between Abu Talha Mansur ibn Sharkab and Ahmad ibn Abdallah al-Khujistani, Hasan sided with the former, but was defeated with him in 878/879, when al-Khujistani recovered Nishapur. Exploiting the turmoils of the period, from 867 Hasan also usually controlled Gurgan to the east, and expanded his control temporarily over some neighbouring regions as well: Rayy (864–865, 867, 870 and 872), Qazvin (865–868) and Qumis (873–879). Relations with the autonomous local Iranian rulers also varied: the Qarinids, who ruled the western mountains of Tabaristan, supported Hasan, but the Bavandids in the eastern mountains were usually hostile, and relations with the Justanid Vahsudan and his son and successor Khurshid also turned hostile. In the event, Hasan managed to have the latter replaced by his brother Justan, who once again loyally supported the Zaydid ruler. The Orientalist Frants Buhl assesses Hasan's character thus: he "possessed rare energy and the capacity for stubborn resistance, was a sincerely religious man, well educated, and a patron of letters".