However, this relationship soon became strained as the trade in slaves became the primary interest and occupation of the Portuguese in Africa.  By 1575 more than 400,000 slaves had been exported, primarily to the plantation-based colonies of Sao Tome and Brazil, as well as to other New World colonies.  The effects of the slave trade severely weakened the Kongo state and removed nearly all vestiges of the friendly relationship and interest in Portuguese religion and culture that existed upon contact.  Until the late nineteenth century Portugal's presence in Angola was mostly limited to the coast, and much of the territory was controlled indirectly through semi-autonomous fiefdoms held by African allies or mesticos of mixed European and African descent.  Because the European population was so small, mesticos, also known as Afro-Portuguese, performed key roles in the military, civil service, and business, occupying a privileged social position above the indigenous Africans and below the Portuguese.