The professional slave soldier, so characteristic of later Islamic empires, was not present in the earliest Islamic regimes. There were indeed slaves who fought in the army of the Prophet, but they were there as Muslims and as loyal followers, not as slaves or professionals. Most of them were freed for their services, and according to an early narrative, when the Prophet appeared before the walls of the Hijaz town of Ta'if, he sent a crier to announce that any slave who came out and joined him would be free. Abu Muslim, the first military leader of the Abbasid revolution which transformed the Islamic state and society in the mid-eighth century, appealed to slaves to come and join him and offered freedom to those who responded. So many, we are told, answered his call that he gave them a separate camp and formed them into a separate combat unit. During the great expansion of the Arab armies and the accompanying spread of the Islamic faith in the seventh and early eighth centuries, many of the peoples of the conquered countries were captured, enslaved, convcrted, and liberated, and great numbers of these joined the armies of Islam. Iranians in the East, Berbers in the West, reinforced the Arab armies and contributed significantly to the further advance of Islam, eastward into Central Asia and beyond, westward across North Africa and into Spain. These were, however, not slaves but freedmen. Though their status was at first inferior to that of freeborn Arabs, it was certainly not servile, and in time the differences in rank, pay, and status between free and freed soldiers disappeared. As so often, the historiographic tradition foreshortens this development and attributes it to a decree of the Caliph 'Umar, who is said to have ordered his governors to make the privileges and duties of manumitted and converted recruits "among the red people" the same as those of the Arabs. "What is due to these, is due to those; what is due from these, is due from those." The limitation of this concession to the "red people," a term commonly applied by the Arabs to the Iranians and later extended to their Central Asian neighbors, is surely significant. The recruitment of aliens, that is, non-Arabs and often non-Muslims, was by no means restricted to liberated captives, and the distinction between freed subjects, free mercenaries, and bought barbarian slaves is often tenuous.