Missouri became a state in 1821 as a slave state. The Missouri Compromise of 1820 admitted Missouri as a slave state and Maine as a free state, and it also prohibited slavery in the remaining Louisiana Territory north of the 36°30′ parallel. The Compromise averted a potential crisis between pro-slavery and anti-slavery factions in the United States Congress, but it also exacerbated tensions between the North and the South and ultimately contributed to the outbreak of the Civil War.
The Missouri Compromise also included a provision that allowed for the gradual emancipation of slaves in Missouri, but this provision was never implemented. In 1861, Missouri seceded from the Union and joined the Confederacy. The state remained a slave state until the end of the Civil War in 1865, when slavery was abolished in the United States.