General Map of the United States, by Henry D. Rogers, 1857
The "line" that delineates free states from slaveholding states was the one agreed upon in the Missouri Compromise of 1820.
Geography and Maps Division, Library of Congress
A Fire Bell in the Night, 1819-1820
When Missouri petitioned to be admitted as a slave state in 1819, it ignited a dispute that Thomas Jefferson compared to "a fire bell in the night." But this was one fire Congress could not put out completely.
Representative James Tallmadge of New York proposed an amendment to Missouri's statehood bill gradually ending slavery there. The Senate defeated the bill because of Tallmadge's amendment. The next year, Senator Jesse Thomas of Illinois devised a compromise: simultaneously admit Missouri as a slave state and Maine as a free state, while banning slavery in most of the Louisiana Territory. Speaker Henry Clay used his popularity and parliamentary skill to win House agreement. The solution ended the immediate crisis—but only postponed a final showdown.
If you persist, the Union will be dissolved. You have kindled a fire which all the waters of the ocean cannot put out, which seas of blood can only extinguish."
—Representative Thomas W. Cobb of Georgia, 1819