Each religion possesses a certain number of primary principles which form its essential basis and other principles of secondary importance. When the followers of a religion differ as to the nature of the primary principles and their secondary aspects but preserve a common basis, the result is called division (inshi'ab) within that religion. Such divisions exist in all traditions and religions, and more particularly in the four "revealed" religions of Judaism, Christianity, Zoroastrianism, and Islam. Shi'ism did not undergo any divisions during the imamate of the first three Imams: Ali, Hasan, and Husayn.
But after the martyrdom of Husayn, the majority of the Shi'ites accepted the imamate of Ali ibn Husayn al-Sajjad, while a minority known as the Kisaniyah believed that the third son of Ali, Muhammad ibn Hanafiyah, was the fourth Imam as well as the promised Mahdi, and that he had gone into occultation in the Radwa mountains and one day would reappear.
After the death of Imam al-Sajjad the majority of the Shi'ites accepted as Imam his son, Muhammad al-Baqir, while a minority followed Zayd al-Shahid, another son of Imam al-Sajjad, and became known as Zaydis. Following Imam Muhammad al-Baqir, the Shi'ites accepted his son Ja'far al-Sadiq as Imam and after the death of Imam Ja'far the majority followed his son Imam Musa al-Kazim as the seventh Imam. However, one group followed the older son of the sixth Imam, Isma'il, who had died while his father was still alive, and when this latter group separated from the majority of Shi'ites it became known as Isma'ilis. Others accepted as Imam either 'Abdollah al-Aftah or Muhammad, both sons of the sixth Imam.
Finally, another party stopped with the sixth Imam himself and considered him as the last Imam. In the same way, after the martyrdom of Imam Musa al-Kazim the majority followed his son, Ali al-Rida, as the eight Imam. However, some stopped with the seventh Imam and became known as the Waqifiyah. From the eighth Imam to the twelfth, whom the majority of the Shi'ites believe to be the promised Mahdi, no division of any importance took place within Shi'ism.
Even if certain events occurred in the form of division, they lasted but a few days and dissolved by themselves. For example, Ja'far, the son of the tenth Imam, claimed to be Imam after the death of his brother, the eleventh Imam. A group of people followed him but scattered in a few days and Ja'far himself did not follow his claim any further. Further more, there are differences between Shi'ites in theological and juridical matters which must not be considered as division in religious schools. Also the Babi and Baha'i sects, which like the Batinis (the Qaramitah) differ in both the principles (usul) and branches (furu') of Islam from the Muslims, should in any sense be considered as branches of Shi'ism.
The sects which separated from the majority of Shi'ites all dissolved within a short period, except two: the Zaydi and the Isma'ili which continue to exist until now. To this day communities of these branches are active in various parts of the world such as the Yemen, India, and Syria. Therefore, we shall limit our discussion to these two branches along with the majority of Shi'ites who are Twelvers. Zaydism and Its Branches The Zaydis are the followers of Zayd al-Shahid, the son of Imam al-Sajjad. Zayd rebelled in 121/737 against the Umayyad caliph Hisham 'Abd al-Malik and a group paid allegiance to him. A battle ensued in Kufa between Zayd and the army of the caliph in which Zayd was killed. The followers of Zayd regard him as the fifth Imam of the Household of the Prophet.
After him his son, Yahya ibn Zayd, who rebelled against the caliph Walid ibn Yazid and was also killed, took his place. After Yahya, Muhammad ibn 'Abdallah and Ibrahim ibn 'Abdallah, who revolted against the Abbasid caliph Mansur al-Dawaniqi and were also killed, were chosen as Imams. Henceforth for some time there was disorder in Zaydi ranks until Nasir al-Utrush, a descendant of the brother of Zayd, arose in Khurasan. Being pursued by the governmental authorities in that region, he fled to Mazandaran, becoming himself Imam.
For some time his descendants continued to rule as Imams in that area. According to Zaydi belief any descendant of Fatimah (the daughter of the Prophet) who begins an uprising in the name of defending the truth may become Imam if he is learned in the religious sciences, ethically pure, courageous and generous.
Yet for some time after Utrush and his descendants there was no Imam who could bring about an insurrection with the sword until recently when, about sixty years ago, Imam Yahya revolted in the Yemen, which had been part of the Ottoman Empire, made it independent, and began to rule there as Imam. His descendants continued to rule in that region as Imams until very recently. At the beginning the Zaydis, like Zayd himself, considered the first two caliphs, Abu Bakr and Umar, as their Imams. But after a while some of them began to delete the name of the first two caliphs from the list of Imams and placed Ali as the first Imam. From what is known of Zaydi beliefs it can be said that in the principles of Islam (usul) they follow a path close to that of the Mu'tazilites, while in the branches or derivative institutions of the law (furu') they apply the jurisprudence of Abu Hanifah, the founder of one of the four Sunni schools of law. They also differ among themselves concerning certain problems.