Shi`ite sect named after Zayd b. Ali, grandson of Husayn.
The Zaydi sect was formed by the followers of Zayd b. Ali, who led an unsuccessful rebellion against the Umayyad caliph Hisham in 740. According to Zaydi political theory, Ali, Hasan and Husayn are the first three rightful Imams; after them, the imamate is open to whomever of their descendants establishes himself through armed rebellion.
The first Zaydi state was established in Tabaristan (northern Iran) in 864; it lasted until the death of its leader at the hand of the Samanids in 928. Forty years later the state was revived in Gilan (north-western Iran) and survived under Hasanid leaders until the 12th century. In Yemen, a Zaydi state was established in 893 by a Hasanid who had originally been invited to mediate between quarrelling Yemeni tribes. A succession of occupations by foreign dynasties beginning in the tenth century occasionally forced the Zaydi imamate to retreat northwards; however, the imamate survived until the death of its last imam in 1962.