By mid-1820, there were twenty-two states in the United States. Eleven allowed slavery, and were called slave states; the other eleven were called free states.
In 1820, Missouri asked to become a state. In Congress, Northerners and Southerners fought about whether Missouri would become a slave state or a free state. Each group had the same number of senators, and wanted Missouri’s senators on their side.
In a compromise, Congress finally decided that Missouri should become a slave state and that the next state, Maine, should be a free state. This agreement, known as the Missouri Compromise, stated that slavery would be prohibited north of 36 degrees north latitude, the westward extension of Missouri’s southern boundary.
By 1850, the United States had grown from twenty-four states to thirty states. Again, there were an equal number (fifteen) of slave states and free states. California wanted to become a free state. The Compromise of 1850 said that California would become a free state; additionally, the Fugitive Slave Act of 1793 was amended to coerce citizens (under threat of fine or imprisonment) to assist in the return of runaway slaves.
As of 1860, the slave-holding areas were Alabama, Arkansas, Delaware, the District of Columbia, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, Missouri, North Carolina, South Carolina, Tennessee, Texas, and Virginia. The free states were Connecticut, Minnesota, Illinois, Indiana, Iowa, Maine, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Vermont, and Wisconsin.
In South Carolina, here were more slaves than white residents. Most of the slaves in he South were owned by about 385,000 people.
By the 1860 census, the number of slaves had grown to 3,953,700 (the total southern population was about 12 million).