In the following decades, the Kingdom of Kongo became a major source of slaves for Portuguese traders and other European powers. The Cantino Atlas of 1502 mentions Kongo as a source of slaves for the island of São Tomé. Slavery had existed in Kongo long before the arrival of the Portuguese, and Afonso's early letters show the evidence of slave markets. They also show the purchase and sale of slaves within the country and his accounts on capturing slaves in war which were given and sold to Portuguese merchants. It is likely that most of the slaves exported to the Portuguese were war captives from Kongo's campaigns of expansion. In addition, the slaving wars helped Afonso consolidate his power in southern and eastern border regions.