As their economies, societies, and cultures diverged in the nineteenth century, Northerners and Southerners expressed different concepts of the American promise and the place of slavery within it. Their differences crystallized into political form in 1846 when David Wilmot proposed banning slavery in any territory won in the Mexican-American War. “As if by magic,” a Boston newspaper observed, “it brought to a head the great question that is about to divide the American people.” Discovery of gold and other precious metals in the West added urgency to the controversy over slavery in the territories. Congress attempted to address the issue with the Compromise of 1850, but the Fugitive Slave Act and the publication of Uncle Tom’s Cabin hardened northern sentiments against slavery and confirmed southern suspicions of northern ill will. The bloody violence that erupted in Kansas in 1856 and the incendiary Dred Scott decision in 1857 further eroded hope for a solution to this momentous question.

During the extended crisis of the Union that stretched from 1846 to 1861, the slavery question intertwined with national politics. The traditional Whig and Democratic parties struggled to hold together as new parties, most notably the Republican Party, emerged. Politicians fixed their attention on the expansion of slavery, but from the beginning Americans recognized that the controversy had less to do with slavery in the territories than with the future of slavery in the nation.

For more than seventy years, statesmen had found compromises that accepted slavery and preserved the Union. But as each section grew increasingly committed to its labor system, Americans discovered that accommodation had limits. In 1859, John Brown’s militant antislavery pushed white Southerners to the edge. In 1860, Lincoln’s election convinced whites in the Lower South that slavery and the society they had built on it were at risk in the Union, and they seceded. But it remained to be seen whether disunion would mean war.