Slavery expands, the divide deepens

The question of where slavery could expand had already caused a crisis once before. When Missouri applied to join the Union as a slave state in 1819, it threatened to tip the balance between free and slave states in Congress; the Missouri Compromise of 1820, engineered by Henry Clay, admitted Missouri as a slave state and Maine as a free state together, and drew a line at 36 degrees 30 minutes latitude across the rest of the Louisiana Purchase land, banning slavery north of it. That line held an uneasy peace for over two decades -- until the Mexican Cession reopened the entire question of slavery in new western territory.

By 1860, nearly 4 million people were enslaved in the United States, most of them forced to labor in the expanding "cotton kingdom" of the Deep South, where the cotton gin (invented 1793) had made growing cotton with enslaved labor enormously profitable. Enslaved people had no legal rights: they could be bought, sold, and separated from their own families at any time, and faced violence for resisting their enslavers' control. Yet enslaved people resisted constantly and in many ways -- slowing their work deliberately, escaping north or to Mexico, and sometimes rising up directly, as in Nat Turner's Rebellion in Virginia in 1831, which was violently suppressed and led southern states to pass even harsher slave codes. Against this backdrop, David Wilmot's 1846 proposal (the Wilmot Proviso) to ban slavery in any land won from Mexico passed the House but failed in the Senate -- a preview of the fight to come over the Mexican Cession.