Zaydi is a branch of Shi‘i Islam that emerged in support of the abortive revolt of Zayd ibn ‘Ali in Kufa against Umayyad rule in 740 CE. Unlike the Twelver or Imami Shi‘is, Zaydis do not unconditionally condemn the first three caliphs who preceded ‘Ali ibn Abi Talib to the leadership of the Muslim community after the Prophet’s death, and they do not consider Sunni Muslims to be infidels. As such, they are often considered moderates. In political terms, however (and again unlike the quietist Twelvers), the Zaydis are militant and insist on armed rebellion against unjust Sunni rule as a religious obligation. They also seek to establish righteous rule under a qualified imam (supreme leader) from the Prophet’s family (ahl al-bayt). Thus, the Zaydi doctrine of the imamate is one of the group’s most distinctive features, and their history is dominated by a number of hugely influential imams.
Zaydi law has set a number of rigorous qualifications for the imamate, the most important of which are religious knowledge (ijtihad) and descent from either of ‘Ali ibn Abi Talib’s two sons, al-Hasan or al-Husayn. Zaydis hold that ‘Ali was the most excellent of men after the Prophet and that he and his two sons were designated by the Prophet as his legatees. After al-Husayn, the imamate could only be established through a summons to allegiance (da’wa) and the armed rising (khuruj) by a qualified candidate. Over the course of time, Zaydis were able to establish several states in two distinct geographical locations: (1) in the Caspian regions of Tabaristan, Daylam, and Gilan; and (2) in the highlands of Yemen. The Zaydis do not follow the legal teachings of the imam after whom they are named (i. e., Zayd ibn ‘Ali) but rather those of certain later imams, two of whom established states. In matters of theology, Zaydis are antideterminist and anti-anthropomorphist. In the Caspian, the first Zaydi state was established in 864 by al-Hasan ibn Zayd, and the last ended in 1526-1527 in eastern Gilan with the conversion of its ruler to Twelver Shi‘ism. Here two rival schools of law emerged: the Qasimiyya (followers of al-Qasim ibn Ibrahim [d. 860]) and the Nasiriyya (followers of al-Nasir al-Hasan ibn ‘Ali al-Utrush [d. 914]). These were ultimately reconciled doctrinally through the adoption of the principle that all mujtahids were correct regardless of their differences.
It is in the Caspian that Zaydi scholars developed a theology that was heavily influenced by Mu‘tazilism, and, through their close connections with the Zaydis of Yemen, they were able to transmit these to the latter community. In the Yemen and at the invitation of the local tribes, Imam al-Hadi Yahya ibn al-Husayn (d. 911; grandson of the aforementioned al-Qasim b. Ibrahim) established a state in Sa‘da in 897, and this lasted, under varying conditions of expansion and contraction, until 1962, with the emergence of the modern republic of Yemen. Al-Hadi’s teachings in law became dominant among the Zaydis of Yemen and ultimately even among the Caspian Zaydis. During the reign of the last great Zaydi state, the Qasimis (1635-1850s), dynastic rule became the norm, and a group of influential Sunni-oriented reformist scholars emerged under Qasimi patronage, causing a Sunnifi-cation of the religious and political environment. The most prominent Sunni-oriented scholar who helped effect this change was Muhammad al-Shawkani (d. 1834).