Parallel histories of the Fatimids and the Abbasids

I recently learned about the striking parallel between the Fatimid Caliphate and the Abbasid Caliphate from an essay entitled "An Interpretation of Fatimid History" by Bernard Lewis, collected in his book, "From Babel to Dragomans: Interpreting the Middle East".  The essay is also online here.

Quoting from the essay, the following is fascinating:

With the coming of the Fatimids in 358/969, the role of Egypt in the Islamic world was vastly increased and totally transformed. The new masters of Egypt were moved by more than personal or dynastic ambition. They were the heads of a great religious movement, which aimed at nothing less than the transformation and renewal of all Islam. As Isma'ili Shi'ites, they refused to offer even token submission to the Abbasid Caliphs, whom they denounced as wrongdoers and usurpers; they and they alone were the true Imams, by descent and by God's choice the sole rightful heads of the whole Islamic community. The Caliphate was therefore theirs by right, and they would take it from the Abbasids as the Abbasids had taken it from the Umayyad.

In preparing the accomplishment of this plan, the Fatimids followed very closely on the pattern set by the Abbasids. Like the Abbasids in their early days, they appealed to all those who felt that the community of Islam had taken a wrong path, and they argued that only an Imam of the house of the Prophet could restore it to the true one. Like the Abbasids again, they created a secret mission, to preach their cause and to organize those who adhered to it. The Abbasids had begun by establishing themselves in the remote province of Khurasan, on the eastern borders of the Empire; the Fatimids, using the same tactics, concentrated their missionary and political effort first in the Yemen, and then in North Africa. The Abbasids had harnessed the warlike Khurasanis to their purposes; the Fatimids mobilized the Berbers. The Abbasids, sweeping westwards from Khurasan, chose a new central province, Iraq, and built themselves a new capital in Baghdad. The Fatimids, advancing eastwards from Tunisia, moved the center to Egypt, and, near the camps and cantonments of Fustat and al-Qata'i', founded a great new imperial metropolis, the city of Cairo. The poet Ibn Hani', in celebrating the victories of al-Mu'izz in Egypt, looks forward in poetic vision to the next and final stages-the invasion of Iraq, the capture of Baghdad, the advance on the ancient highway to the East.

   At this point, however, the resemblance ceases, for the vision was not fulfilled. The Abbasid triumph was complete, that of the Fatimids only partial. Except for the distant and isolated province of Spain, all Islam submitted to the 'Abbasids, and even in Spain the Umayyad survivors did not seriously challenge their Caliphate. The Fatimids won great victories, and at the time it must have seemed that they were about to engulf the whole world of Islam. But they did not. The Abbasids, defeated and weakened, themselves under the domination of a Shi'ite though not Ismaili dynasty of mayors of the palace, nevertheless managed to hold on in their old capital, and served as a rallying point for all forces of Sunni Islam. In the following century, those forces were immensely strengthened by the advent of the Seljuk Turks and the creation of a new and powerful military empire in the East, the great Sultanate. The reinforcement was religious as well as political. The Seljuk Sultans were devout Sunnis. True, they dominated the Caliphate, but unlike the Sh'ite Buyids whom they replaced, they treated the Caliphs with honor and respect as the supreme religious authority in Sunni Islam, and their advent greatly increased the prestige and influence of the Abbasid house. The containment of the Fatimid danger was not achieved by military and political means alone, though these were essential and in large measure successful. In the madrasa, Sunni Islam created a new and crucial weapon in the struggle for religious unity. In these great colleges, spreading all over the East, the scholars and theologians of the Sunna devised and taught the orthodox answer to the Isma'ili intellectual challenge.