Complex sentence: The separation of the tribes of this region into warrior and Zawāyā tribes had probably occurred before the fifteenth century, when the Beni Ḥassān Arab nomads began to enter the region.
Split into simpler sentences: The separation of the tribes of this region into warrior and Zawāyā tribes had probably occurred before the fifteenth century. During the fifteenth century the Beni Ḥassān Arab nomads began to enter the region.