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The issue that caused the division of the north and south, the conflict of the Compromise of 1850, the Underground Railroad, Harriet Beecher Stowe to write "Uncle Tom's Cabin", the "Bleeding Kansas" conflict, the formation of the Know-Nothing Party and the Free-Soilers, the Dredd Scott decision, John Brown's raid on Harper's Ferry, and ultimately the south's seccession and the Civil War was this.
What is slavery?