After Muhammad's entry into Mecca the tribes linked with Quraysh came to negotiate with him and to accept Islam; this meant little more than giving up their local deities and worshiping Allah alone.

They had to pay the tax, but this was not novel because the tribal chiefs had already been taxed to protect the Meccan haram.

Many tribesmen probably waited to join the winner.

Doubtless they cared little for Islam--many tried to break away (the so-called apostasy) on Muhammad's death.

Islam, however, was destined for a world role.

Under Muhammad's successors the expansionist urge of the tribes, temporarily united around the nucleus of the two sacred enclaves, coincided with the decline and weakness of Byzantium and Sasanian Persia.

Tribes summoned to the banners of Islam launched a career of conquest that promised to satisfy the mandate of their new faith as well as the desire for booty and lands.

With families and flocks, they left the Arabian peninsula.

Population movements of such magnitude affected all of Arabia; in Hadhramaut they possibly caused neglect of irrigation works, resulting in erosion of fertile lands.

In Oman, too, when Arab tribes evicted the Persian ruling class, its complex irrigation system seems to have suffered severely.

Many Omani Arabs about the mid-7th century left for Basra (in Iraq) and formed the influential Azd group there.

Arabian Islam replaced Persian influence in the Bahrain district and Al-Hasa province in the northeast, and in Yemen.

As the conquests far beyond Arabia poured loot into the Holy Cities (Mecca and Medina), they became wealthy centres of a sophisticated Arabian culture; Medina became a centre for Qur'anic study, the evolution of Islamic law, and historical record.

Under the caliphs, Muhammad's successors, Islam began to assume its characteristic shape; paradoxically, outside the cities it made little difference to Arabian life for centuries.

Shari'ah (Islamic law), promoted often by the Prophet's own descendants, developed in the urban centres; but outside them customary law persisted, sometimes diametrically opposed to Shari'ah. In time the Hejaz and Yemen came to make notable contributions to Islamic culture, but Islam's basically Arabian nature first shows in the early mosque, which resembles the pre-Islamic temple, and in the pilgrimage rites, little altered from paganism.