Their combined forces now marched against Nasr, whom they defeated in a pitched battle. Nasr fled to Nīshāpūr, while the allies occupied Merv, where, however, dissensions arose between them which cost Hārith his life, A.H. 128 (745).[171] It was in the midst of these disorders that Abū Muslim, the virtual founder of the `Abbasid dynasty, raised the black banner in Khorāsān. The advent of the `Abbāsids to the Caliphate was an event of such moment for the future of Central Asia that it is necessary in this place to give a brief account of the rise of the new dynasty. The fall of the Umayyads was the death-knell of unity in Islām. In spite of numberless rebellions in all parts of their conquered provinces the Umayyads had never recognised independent rulers, but with the establishment of the house of `Abbās there set in a general dismemberment of the empire of the Caliphs. The origin of the dispute between the Hāshimites (or `Abbāsids) and the Umayyads dates back to a period anterior to the birth of Mohammed. It was a rivalry between the two chief stocks of the house of Koraysh.[172] We have seen above that, although Mohammed, on first declaring his mission, met with opposition from his own tribe, after the conquest of Mekka they temporarily reconciled the conflicting interests. So after the Prophet’s death discussions again arose between `Alī and the Caliph Mo`āwiya. The Khārijites, who demanded a purely theocratic rule, were also continually in a ferment. After the tragic death of Husayn, the son of `Alī, at Kerbelā, a party arose devoted to the house of `Alī, and claiming the succession of his family to the Caliphate, who called themselves the Shī`a (or faction), and who are known to Europeans as the Shi`ites. In the reign of Hishām (A.H. 105), Mohammed, the great-grandson of the Prophet’s uncle, `Abbās, who was living in retreat in the south of Palestine, began to advance his claims to the Caliphate. Emissaries and secret deputations were sent to all the principal towns of Persia, `Irāk, and Khorāsān, and, in spite of the severe measures taken to check the movement, the cause of the Hāshimites began rapidly to spread. The Shi`ites and the Khārijites were induced to make common cause with the Hāshimites, on the plea that the only object of the movement was to secure the Caliphate for a member of the Prophet’s own family.