As the Houthi Movement in Yemen seems about ready to remove President al-Hadi, many Western commentators seem uncertain what to make of them. As I noted as far back as 2009, referring to these Zaydi Muslim revivalists as "Shi‘ite" without qualification is misleading, since their Shi‘ism is vastly different from that of Iran. Because Saudi Arabia suspects they have Iranian support, and they seem to, they are often seen as Iranian stooges, but in fact they are very much a home-grown movement springing from the Zaydi tribes of the North Yemen mountains.
Starting as a Zaydi revivalist youth movement, the Houthis are fiercely anti-Western and have opposed the GCC-brokered transition plan under way in Yemen since 2011.
I'll leave it to the Yemen specialists to explain the radicalism of Houthi ideology; in the meantime I want to note that Zaydism as a religious school has been strikingly accommodating to Sunnis and other sects of Shi‘ism as well. With the Houthis, though, even Zaydism has acquired a radical face, though not all Zaydis back the Houthis, of course.
Zaydism is indeed a branch of Shi‘ism, the second-largest after the "Twelvers" of Iran, Iraq and Lebanon. But its history and beliefs are quite different from other varieties. While they are called "Fivers" because they accept the first four Imams recognized by Twelvers and Isma‘ilis ("Seveners") and recognize Zayd ibn ‘Ali as the rightful successor to his father ‘Ali Zayn al-‘Abidin while other groups recognize his brother Muhammad al-Baqir, they do not then insist that all rightful imams must descend from Zayd. In fact, the Zaydi doctrine of the Imamate differs enormously from other Shi‘a.
The Zaydis hold that ‘Ali, the first Imam, was the rightful successor to Muhammad, but they hold that since the Prophet did not make the succession clear and ‘Ali did not press his claim, they do not publicly curse Abu Bakr and ‘Umar. They hold that ‘Ali and his two sons, Hasan and Husayn, were rightful Imams, but that after them, the Imamate could be intrusted to any just, wise ruler descended from Hasan or Husayn, not limited to a single line. Though they recognize Zayd as their fifth Imam and his son Muhammad as their sixth, their list includes others from the Twelver line as well. There were various early sects of Zaydism that disagreed on some of these points, however.
The Zaydis do not believe in the infallibility of the Imams after the first three, and thus they also accept that geographically distant parts of the world could follow different lines of Imams. In the medieval period, there were Zaydi imamates in Tabaristan in northeast Iran and Gilan in northwest Iran as well as in Yemen. Zaydi states at one time also existed in the Arabian Peninsula outside Yemen, and in Morocco and Spain.
The Zaydi legal school is very similar to that of Abu Hanifa in Sunnism,
and some have listed Zaydi law as a "fifth school" of Sunnism, except
for the doctrine of the Imamate.
Zaydism was established in the highlands of Yemen in 893 AD and an imamate continued under various lines until the Revolution of 1962. In the 20th century the Imams also added the title of King and the title "Mutawakkilite Kingdom of Yemen" for the country.
The last Imam, Muhammad al-Badr, continued to fight with Saudi backing in the eight year civil war, in which Nasser's Egypt backed the republicans. In 1970 he went into exile in Britain. He died in 1996. With the absence of any claimant to the Zaydi Imamate in Yemen, the distinctions between Zaydis and Sunnis became even fewer.
In former North Yemen, Zaydis were long a majority in the mountain interior, while the coastal plain was mostly Sunni of the Shaf‘i school. Unification with South Yemen in 1990 made Zaydism a minority (about 40%) in the country as a whole.
This tradition of there being little perceived difference between Zaydis and Sunnis seems to be another casualty of recent events in Yemen, given the radicalism of the Houthis.