Quoting from the essay, the following is fascinating:
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.