The biggest slave population in the United States of the time of the Civil War was most likely Virginia. Virginia was known for having a lot of slaves and bringing them over from Africa. So this would be considered the epicenter of slavery. The reason Virginia had so many slaves is due to their plantations. In order to grow crops, the climate would have to be not too hot or not too cold.
Virginia had the right type of climate even though there were some other states that have large plantations and the number of slaves running them. However Virginia had the most. Many famous leaders who came from Virginia were known to have plantations and some of them had slaves.