From Wikipedia

Slavery And The Making Of America
Episode Three

Seeds Of Destruction

Slavery was the major issue of the American Civil War in which the Union emerged victorious, after which the slave-labor system was abolished in the South. This contributed to the decline of the postbellum Southern economy, but it was most affected by the continuing decline in the price of cotton through the end of the century. That made it difficult for the region to recover from the war, as did its comparative lack of infrastructure, which kept products from markets. The South faced significant new competition from foreign cotton producers such as India and Egypt. Northern industry, which had expanded rapidly before and during the war, surged even further ahead of the South's agricultural economy. Industrialists from northeastern states came to dominate many aspects of the nation's life, including social and some aspects of political affairs. The planter class of the South lost power temporarily. The rapid economic development following the Civil War accelerated the development of the modern U.S. industrial economy.

Twelve million Africans were shipped to the Americas from the 16th to the 19th centuries. Of these, an estimated 645,000 were brought to what is now the United States. The largest number were shipped to Brazil (see slavery in Brazil). The slave population in the United States had grown to four million by the 1860 Census.