The Umayyads continued the Muslim conquests, incorporating the Transoxiana, Sindh, the Maghreb and the Iberian Peninsula (Al-Andalus) into the Muslim world. At its greatest extent, the Umayyad Caliphate covered 11,100,000 km2 (4,300,000 sq mi) and 62 million people (29% of the world's population), making it one of the largest empires in history in both area and proportion of the world's population.
The Umayyad Caliphate was de facto secular. At the time, the Umayyad taxation and administrative practice were perceived as unjust by some Muslims. The Christian and Jewish population still had autonomy; their judicial matters were dealt with in accordance with their own laws and by their own religious heads or their appointees, although they did pay a poll tax (the jizya) for protection to the central state. Muhammad had stated explicitly during his lifetime that Abrahamic religious groups (still a majority in times of the Umayyad Caliphate) should be allowed to practice their own religion, provided that they paid the jizya taxation. The welfare state of both the Muslim and the non-Muslim poor started by Umar ibn al Khattab had also continued, financed by the Zakat tax levied only on Muslims.
Muawiya's wife Maysum (Yazid's mother) was also a Christian. The relations between the Muslims and the Christians in the state were stable in this time. The Umayyads were involved in frequent battles with the Christian Byzantines without being concerned with protecting themselves in Syria, which had remained largely Christian like many other parts of the empire. Prominent positions were held by Christians, some of whom belonged to families that had served in Byzantine governments. The employment of Christians was part of a broader policy of religious assimilation that was necessitated by the presence of large Christian populations in the conquered provinces, as in Syria. This policy also boosted Muawiya's popularity and solidified Syria as his power base.
According to tradition, the Umayyad family (also known as the Banu Abd-Shams) and Muhammad both descended from a common ancestor, Abd Manaf ibn Qusai, and they originally came from the city of Mecca in the Hijaz. Muhammad descended from Abd Manāf via his son Hashim, while the Umayyads descended from Abd Manaf via a different son, Abd-Shams, whose son was Umayya. The two families are therefore considered to be different clans (those of Hashim and of Umayya, respectively) of the same tribe (that of the Quraish). However Muslim Shia historians suspect that Umayya was an adopted son of Abd Shams and thus not a blood relative of Abd Manaf ibn Qusai. Umayya was later discarded from the noble family.
Sunni historians disagree with this and view Shia claims as nothing more than outright polemics due to Shia hostility to the Umayyad family in general. They point to the fact that the grandsons of Uthman, Zaid bin Amr bin Uthman bin Affan and Abdullah bin Amr bin Uthman got married to Sukaina and Fatima (according to a Sunni reference only), the daughters of Hussein son of Ali, to show the closeness of the Banu Hashim and the Bani Ummayah.