 Complete the following phrase:  Contrary to the belief of earlier historians, there is no evidence of mass conversions to Islam in the immediate aftermath of the conquests. The first groups to convert were Christian Arab tribes, although some of them retained their religion into the Abbasid era even while serving as troops of the caliphate. They were followed by former elites of the Sasanian empire, whose conversion ratified their old privileges. With time, the weakening of non-Muslim elites facilitated the breakdown of old communal ties and reinforced the incentives of conversion which promised economic advantages and social mobility. By the beginning of the eighth century, conversions became a policy issue for the caliphate. They were favored by religious activists, and many Arabs accepted the equality of Arabs and non-Arabs. However, conversion was associated with economic and political advantages, and Muslim elites were reluctant to see their privileges diluted. Public policy towards converts varied depending on the region and was changed by successive Umayyad caliphs. These circumstances provoked opposition from non-Arab converts, whose ranks included many active soldiers, and helped set the stage for the civil war which ended with the 