The Arabs, within the two decades which followed the death of the Prophet Muhammad, won for themselves a large empire embracing Syria, Egypt, Iraq, Persia and much of Arabia itself. The role, in the armies of Islam, of soldiers Muslim through conversion and non-Arab in ethnic origin grew in importance during the years of Umayyad rule. The pattern of warfare which had brought the Arabs success in the time of the great conquests was soon overlaid, as it were, with procedures drawn from the traditions of Byzantium and Persia. The Arabs who conquered a great empire for Islam had little acquaintance with the techniques of siege warfare. The period of the Abbasid decline saw a large increase in the use of mamluks recruited as slaves, trained in the practice of war and freed to serve as professional troops. The iqta system was reaching its full development in the sixth/twelfth and seventh/thirteenth centuries.