These citystates began to decline in the sixteenth century; the advent of Portugese trade disrupted the old trade routes and made the Swahili commercial centers obsolete. The Portugese wanted native Africans to have no share in African trade and busily set about conquering the Islamic citystates along the eastern coast. In the late seventeenth century, Oman in the south of Arabia then conquered all the Portugese cities along the coast and the eastern African coast was controlled by the Omani sultanate for another two hundred years.

Website: http://www.wsu.edu:8080/~dee/CIVAFRCA/SWAHILI.HTM