The first African slaves arrived in Virginia sometime during the seventeenth century. In its early days, though, American slavery was not confined to any particular region. It was legal almost everywhere; indeed the first American slave ships set sail from Massachusetts.

But by the time of the American Revolution, slavery was no longer widespread north of Maryland. It had become largely a Southern institution. Soon after the founding of the United States, slavery became illegal throughout the North, yet it was flourishing across the South. Over the next several decades, the population of the South expanded to the west, and so did slavery. By 1861, when the Civil War began, the slave states stretched from Delaware to Texas and from Missouri to Florida.

Today, Americans often picture the prewar South as a place where nearly every white person lived on a large plantation surrounded by dozens of...