The history of human civilization shows that successful growth requires manpower, and where societies were ruled by force, they could compel some individuals to work under bondage. When large or advanced groups encountered smaller or weaker groups, they would often dominate and enslave its people (not having any moral injunction against this as they might for their own families or citizens).
More easily enforced was slavery of other ethnic or racial groups, where the slaves were more or less identifiable (Jews in Egypt, Greeks and others in Rome, the various competing tribes in Africa). This became eminently the case with black slaves in white European colonies. African slaves were imported to the West Indies after the natives (Indians) proved intractable and rebellious. Black slavery did not long succeed either, due to their majority in the total population, and the Hispaniola colonies threw out French rule.
Only in the US South did slavery as an institution survive a local disparity in numbers. Brought to America as part of the triangle trade (molasses, rum, and slaves), they were mostly owned by large plantations. New generations were taught from birth that they were property, and the few rebels were harshly punished or killed as object lessons. In the North, with its large immigrant populations, slavery was generally less important economically, and over time many slaves were freed. The secession acts of some Southern states showed that their slaveowners felt threatened with the loss of their one economic advantage over the North. So while Britain outlawed slavery in 1807, and France in 1814 (post-Napoleon), it took the Civil War to eliminate legal slavery in the US. The 13th amendment to the Constitution (1865) affirmed the rights of all in the US to be free.
With the rise of trade unions, "economic slavery" was also reduced in the West, but legal or illegal bondage still exists in many parts of the world, particularly in Arabic states. And women around the world are still forced into prostitution ("white slavery").
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In the beginning, southern plantation owners used Africans as slaves because Africans knew how to plant rice. However, the most popular reason for slavery originated in the South because of the plantations and farms. The plantation owners and farmers needed field hands to help them cultivate their farms. Once Eli Whitney invented the cotton gin, the number of slaves in the United States rapidly increased because the cotton gin increased the production of cotton.
Slavery happened because of the following reasons: 1. because the African people were willing to sell there own. 2. African people had not invented corporations.