Soon after 1600, with the area mostly united under one native ruler, native tribesmen began encountering, trading with and making alliances with Portuguese traders and members of the military. By 1700, however, the empire had broken up into areas controlled by many individual ethnic groups. The Indian Ocean slave trade reached its height in the mid-1800s, when approximately 20,000 people were enslaved and considered to be carried yearly from Nkhotakota to Kilwa where they were sold.