The Abolitionist movement in the United States had roots in the Declaration of Independence. Slavery was banned in the Northwest Territory with the Northwest Ordinance of 1787. By 1804 all the Northern states had passed laws to abolish slavery. Congress banned the African slave-trade in 1808, although slavery grew in new states in the deep south. The Union was divided along the Mason Dixon Line into the North (free of slaves), and the South, where slavery remained legal. This is one of the causes of American Civil War.

Despite compromises in 1820 and 1850, the slavery issues exploded in the 1850s. 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. 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 resentment of the influence that the Slave Power already wielded in government, brought the crisis to a head in the late 1850s. Disagreements between Abolitionists and others over the morality of slavery, the scope of democracy and the economic merits of free labor versus 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.

Northerners ranging from the abolitionist William Lloyd Garrison to the moderate Republican leader Lincoln emphasized Jefferson's declaration that all men are created equal. Lincoln mentioned this proposition many times, including his 1863 Gettysburg Address.

Almost all the inter-regional crises involved slavery, starting with debates on the three-fifths clause and a twenty-year extension of the African slave trade in the Constitutional Convention of 1787. The 1793 invention of the cotton gin by Eli Whitney increased by fiftyfold the quantity of cotton that could be processed in a day and greatly increased the demand for slave labor in the South. There was controversy over adding the slave state of Missouri to the Union that led to the Missouri Compromise of 1820. A gag rule prevented discussion in Congress of petitions for ending slavery from 1835–1844, while Manifest Destiny became an argument for gaining new territories, where slavery could expand. The acquisition of Texas as a slave state in 1845 along with territories won as a result of the Mexican–American War (1846–1848) resulted in the Compromise of 1850. The Wilmot Proviso was an attempt by Northern politicians to exclude slavery from the territories conquered from Mexico. The extremely popular antislavery novel Uncle Tom’s Cabin (1852) by Harriet Beecher Stowe greatly increased Northern opposition to the Fugitive Slave Law of 1850.

The 1854 Ostend Manifesto was an unsuccessful Southern attempt to annex Cuba as a slave state. The Second Party System broke down after passage of the Kansas-Nebraska Act in 1854, which replaced the Missouri Compromise ban on slavery with popular sovereignty, allowing the people of a territory to vote for or against slavery. The Bleeding Kansas controversy over the status of slavery in the Kansas Territory included massive vote fraud perpetrated by Missouri pro-slavery Border Ruffians. Vote fraud led pro-South Presidents Franklin Pierce and James Buchanan to attempt to admit Kansas as a slave state. Buchanan supported the pro-slavery Lecompton Constitution.

Violence over the status of slavery in Kansas erupted with the Wakarusa War, the Sacking of Lawrence, the caning of Republican Charles Sumner by the Southerner Preston Brooks, the Pottawatomie Massacre, the Battle of Black Jack, the Battle of Osawatomie and the Marais des Cygnes massacre. The 1857 Supreme Court Dred Scott decision allowed slavery in the territories even where the majority opposed slavery, including Kansas.

The Lincoln-Douglas debates of 1858 included Northern Democratic leader Stephen A. Douglas' Freeport Doctrine. This doctrine was an argument for thwarting the Dred Scott decision that, along with Douglas' defeat of the Lecompton Constitution, divided the Democratic Party between North and South. Northern abolitionist John Brown's raid at Harpers Ferry Armory was an attempt to incite slave insurrections in 1859. The North-South split in the Democratic Party in 1860 due to the Southern demand for a slave code for the territories completed polarization of the nation between North and South.

Causes of American Civil War