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
When Missouri, which allowed slavery, applied for statehood in 1819, Congress struggled for a way to maintain the Union despite strongly opposing pro- and antislavery constituencies. After heated debate, Congress adopted the Missouri Compromise, which admitted Maine as a free state to balance Missouri and prohibited slavery north of the 36º 30’ latitude in the Louisiana Territory.