The slave trading ports of West Africa were readily accesible to European traders, obviously because they were coastal, and also because they were the closest to Western Europe.
In West Africa at the time, there were many warring tribes. These tribes were rivals to each other, and when one tribe overrode another, if often subjugated the defeated to slavery. Europeans entered the equation by negotiating with some Africans for the buying and selling of other africans as slaves.
