Beginning with the Arab conquest of the western Maghrib in the eighth century, Mauritania experienced a slow but constant infiltration of Arabs and Arab influence from the north. The growing Arab presence pressed the Berbers, who chose not to mix with other groups, to move farther south into Mauritania, forcing out the black inhabitants. By the sixteenth century, most blacks had been pushed to the Senegal River. Those remaining in the north became slaves cultivating the oases.

After the decline of the Almoravid Empire, a long process of arabization began in Mauritania, one that until then had been resisted successfully by the Berbers.
Several groups of Yemeni Arabs who had been devastating the north of Africa turned south to Mauritania. Settling in northern Mauritania, they disrupted the caravan trade, causing routes to shift east, which in turn led to the gradual decline of Mauritania's trading towns. One particular Yemeni group, the Bani Hassan, continued to migrate southward until, by the end of the seventeenth century, they dominated the entire country. The last effort of the Berbers to shake off the Arab yoke was the Mauritanian Thirty Years' War (1644-74), or Sharr Bubba, led by Nasir ad Din, a Lemtuna imam.
This Sanhadja war of liberation was, however, unsuccessful ; the Berbers were forced to abandon the sword and became vassals to the warrior Arab groups.

Thus, the contemporary social structure of Mauritania can be dated from 1674. The warrior groups or Arabs dominated the Berber groups, who turned to clericalism to regain a degree of ascendancy. At the bottom of the social structure were the slaves, subservient to both warriors and Islamic holy men. All of these groups, whose language was Hassaniya Arabic, became known as Maures. The bitter rivalries and resentments characteristic of their social structure were later fully exploited by the French.