The coexistence of a slave-owning South with an increasingly anti-slavery North made secession inevitable. Lincoln did not propose federal laws against slavery where it already existed, but he had, in his 1858 House Divided Speech, expressed a desire to "arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction". Much of the political battle in the 1850s focused on the expansion of slavery into the newly created territories. All of the organized territories were likely to become free-soil states, which increased the Southern movement toward secession. Both North and South assumed that if slavery could not expand it would wither and die.
Southern fears of losing control of the federal government to antislavery forces, and Northern fears that the slave power already controlled the government, brought the crisis to a head in the late 1850s. Sectional disagreements over the morality of slavery, the scope of democracy and the economic merits of free labor vs. slave plantations caused the Whig and "Know-Nothing" parties to collapse, and new ones to arise (the Free Soil Party in 1848, the Republicans in 1854, the Constitutional Union in 1860). In 1860, the last remaining national political party, the Democratic Party, split along sectional lines.
Both North and South were influenced by the ideas of Thomas Jefferson. Southerners emphasized the states' rights ideas mentioned in Jefferson's Kentucky and Virginia Resolutions and the right of revolution mentioned in the Declaration of Independence. Northerners ranging from the abolitionist William Lloyd Garrison to the moderate Republican leader Abraham Lincoln emphasized Jefferson's declaration that all men are created equal. Lincoln mentioned this proposition in his Gettysburg Address.
Historian Kenneth M. Stampp mentioned Confederate Vice President Alexander Stephens' A Constitutional View of the Late War Between the States as an example of a Southern leader who said that slavery was the "cornerstone of the Confederacy" when the war began and then said that the war was not about slavery but states' rights after Southern defeat. Stampp said that Stephens became one of the most ardent defenders of the Lost Cause.
All but one inter-regional crisis involved slavery, starting with debates on the three-fifths clause in the Constitutional Convention of 1787. Other factors include modernization in the rapidly industrializing North, sectionalism (caused by the growth of slavery in the deep South while slavery was gradually phased out in Northern states) and economic differences between North and South, although most modern historians disagree with the extreme economic determinism of historian Charles Beard. The fact that seven immigrants out of eight settled in the North, plus the fact that twice as many whites left the South for the North as vice versa, contributed to the South's defensive-aggressive political behavior There was controversy over adding the slave state of Missouri to the Union that led to the Missouri Compromise of 1820, the Nullification Crisis over the Tariff of 1828 (although the tariff was low after 1846 ), the Gag rule that prevented discussion in Congress of petitions for ending slavery from 1835–1844, the acquisition of Texas as a slave state in 1845 and Manifest Destiny as an argument for gaining new territories where slavery would become an issue after the Mexican-American War (1846–1848), which resulted in the Compromise of 1850. The Wilmot Proviso was an unsuccessful attempt by Northern politicians to exclude slavery from the territories conquered from Mexico. There were unsuccessful attempts to end controversy over slavery in the territories through popular sovereignty and Southern attempts to annex Cuba (including the Ostend Manifesto) and Nicaragua as slave states. The extremely popular antislavery novel Uncle Tom’s Cabin (1852) b