structural:expansion
New Arab settlements

The society of new Arab settlements gradually became stratified into classes based on wealth and power. It was also reorganized into new communal units that preserved clan and tribal names but were in fact only loosely based around old kinship bonds. Arab settlers turned to civilian occupations and in eastern regions established themselves as a landed aristocracy. At the same time, distinctions between the conquerors and local populations began to blur. In Iran, the Arabs largely assimilated into local culture, adopting the Persian language and customs and marrying Persian women. In Iraq, non-Arab settlers flocked to garrison towns. Soldiers and administrators of the old regime came to seek their fortunes with the new masters, while slaves, laborers and peasants fled there seeking to escape the harsh conditions of life in the countryside. Non-Arab converts to Islam were absorbed into the Arab-Muslim society through an adaptation of the tribal Arabian institution of clientage, in which protection of the powerful was exchanged for loyalty of the subordinates. The clients (mawali) and their heirs were regarded as virtual members of the clan. The clans became increasingly economically and socially stratified. For example, while the noble clans of the Tamim tribe acquired Persian cavalry units as their mawali, other clans of the same tribe had slave laborers as theirs. Slaves often became mawali of their former masters when they were freed.

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 8th 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 fall of the Umayyad dynasty.