General Map of the United States, by Henry D. Rogers, 1857
A map published nearly forty years after the Missouri Compromise distinguished free states (dark green) and free territories (light green) from slave-importing (light red) and slave-exporting (dark red) states.
Geography and Map Division, Library of Congress
The Missouri Compromise
To achieve national unity, the country’s founders did not confront the issue of slavery in the Constitution, but left it for future generations to resolve. In the nineteenth century, applications for statehood from western territories threatened the balance of free and slave states in the Senate. Congress shifted the basis for determining the status of slavery in each territory from the territory’s geographical location to a popular vote by its inhabitants. Slavery remained a burning issue that threatened the Union and led to the Civil War.