How did the modern system of slavery develop?

 We often think of past slavery as being associated with racial beliefs however, the origins of this institutions do not seem to be associated with race. Slavery at times has been used as a form of punishment or to deal with prisoners of war.

Slavery used to be a part of the tradition of many tribes in the past. Slaves were usually given or they were prisoners of war. It develop more when the Europeans came to the US. They have started investing on tobacco plantations and other agricultural crops. They would often buy slaves to help them tend the lands and at the same time, they also hire indentured slaves to make the work faster and easier. Because of this, many tribes in Africa would sell slaves to earn more gold. Many African slaves were brought around the different colonies of Britain, Dutch Indies, Spain and Portugal. 

The answer is crop yield.