During the 19th century, the United States became embroiled in the debate over the legality and morality of slavery. The states began to become known as being either pro-slavery or anti-slavery, and this division was one of the reasons the Civil War occurred. During that era, the United States was composed of fewer states than it is today; the states to the north were generally considered to be anti-slavery, while southern states were considered to be pro-slavery. Up until the early 1800s, almost all the states allowed slavery at some point.

During the early 19th century, more pro-slavery states existed in relation to the total number of states. As that century progressed, more states became anti-slavery, but more states were also admitted to the Union. By the time the Civil War began, all of the New England states — which included Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut — were anti-slavery states, as were New York, Pennsylvania, New Jersey, Ohio, Indiana, Michigan, Illinois, Wisconsin, Iowa, and Minnesota. On the west coast, California and Oregon were anti-slavery, as were some of the territories in between.

Pro-slavery states were south of these anti-slavery states. Maryland and Delaware were the northernmost pro-slavery states at the start of the Civil War, and all of the states south of those two states were considered pro-slavery as well. Missouri was also fairly northern, and it was a state that allowed slavery as well. Richmond, a city in Virginia, became the capital of the Confederacy, which fought for slavery, among other political reasons. Kentucky, where President Abraham Lincoln was born, was a state in favor of slavery.

North Carolina, South Carolina, Georgia, Florida, Mississippi, Alabama, Louisiana, Tennessee, Arkansas, Texas, the Indiana Territory, and the New Mexico Territory were all considered pro-slavery states or regions of the country at the start of the Civil War. Some northern states were slave states before the 19th century and shortly into the 20th century, but they quickly became anti-slavery states well before the Civil War erupted in 1861. While slavery was not the only issue to drive the country to war, it was certainly a major contributing factor.

After the Civil War ended with a Union victory, a constitutional amendment was written to abolish slavery throughout the United States. All states were then considered anti-slavery regardless of which side those states had chosen during the war. This happened in 1865, several months after Lincoln was assassinated.