Zaidism's political history is the longest of any Shia tradition. Imamates were established in northern Iran from the ninth century, and the Idrisid dynasty of Morocco, from 788, is often counted among them. The enduring one was in Yemen.
Al-Hadi ila'l-Haqq Yahya was invited to arbitrate among the tribes of the northern highlands and established an imamate at Sa'dah at the end of the ninth century. With interruptions, conquests and rival claimants, that institution lasted until 1962, a religious polity of a thousand years, longer than any other Shia state has managed. Its learning was carried in protected scholarly settlements, and in the sada, families claiming descent from the Prophet, whose social position the imamate underwrote.
It ended abruptly. A revolution in 1962 abolished the imamate and proclaimed the Yemen Arab Republic, and the civil war that followed ran to 1970. With the imamate went the institutional structure of Zaidism. There is no Zaidi religious authority today: no imam, no hierarchy, no equivalent of the seminaries of Najaf or Qom. Authority survives informally, among scholarly families around Sa'dah.
The republic that replaced the imamate was not neutral toward the tradition. Saudi-funded Salafi institutions were established in the Zaidi heartland from the 1980s, teaching that Zaidi practice was heterodox, and the sense of a tradition losing ground in its own territory is the direct background to what followed. A thousand-year institution had been abolished, and nothing had been put in its place.