From Slaveryinamerica.org:
On the eve of the American Civil War approximately 4 million enslaved African Americans lived in the southern region of the United States of America. The vast majority worked as plantation slaves in the production of cotton, sugar, tobacco, and rice. Very few of these enslaved people were African born principally because the importation of enslaved Africans to the United States officially ended in 1808, although thousands were smuggled into the nation illegally in the 50 years following the ban on the international trade. These enslaved people were the descendants of 12 to 13 million African forbearers ripped from their homes and forcibly transported to the Americas in a massive slave trade dating from the 1400s. Most of these people, if they survived the brutal passages from Africa, ended up in the Caribbean (West Indies) or in South and Central America. Brazil alone imported around five million enslaved Africans. This forced migration is known today as the African Diaspora, and it is one of the greatest human tragedies in the history of the world.
From the beginnings of slavery in British North America around 1619, when a Dutch ship brought 20 enslaved Africans to the Virginia colony at Jamestown, nearly 240 years passed until the Thirteenth Amendment to the U. S. Constitution officially ended slavery in 1865. This means that 12 generations of blacks survived and lived in America as enslaved people-direct descendants of the nearly 500,000 enslaved Africans imported into North America by European traders. Some of the 180,000 African Americans who fought for their freedom as Union soldiers in the American Civil War could trace their families to the time of the Pilgrims. Some of their family histories in America predated those of Thomas Jefferson, George Washington, Abraham Lincoln, and most sitting members of Congress and the U. S. Senate in 1860.
More here.
