Within a generation of their crossing the border of the peninsula in a.d. 632, the Arab armies conquered most of the territories of the Byzantine Empire and destroyed the Sassanid Empire. In the following century they added substantial territories in the East and particularly in the West so that their empire in the early eighth century extended from the Indus to the Provence. As a result of the replacement of the Omayyad by the Abbasid dynasty in a.d. 752, most of North Africa as well as Spain became independent, in fact reducing the population of the caliphate by about one-fourth and shifting its political and economic center from Syria to Mesopotamia and Iran and the capital from Damascus to Baghdad. The conquest had been so rapid, the number of conquerors so small, and the economic and cultural level of the conquered territory so much higher than that of Arabia that the caliphate in most fields took over the existing Byzantine or Sassanid institutions with only little change, the most important one being the conversion of a substantial part of the population to Islam and from the eighth century on the use of Arabic as the sole official language. This was done to such an extent that the Omayyad caliphate has been called the neo-Byzantine Empire and the Abbasid caliphate the neo-Sassanid Empire.