Between 1819 and 1860, the critical issue that divided the North and South was the extension of slavery in the western territories. The Compromise of 1820 had settled this issue for nearly 30 years by drawing a dividing line across the Louisiana Purchase that prohibited slavery north of the line, but permitted slavery south of it.

The seizure of new territories from Mexico reignited the issue.  The Compromise of 1850 attempted to settle the problem by admitting California as a free state but allowing slavery in the rest of the Mexican cession.  Enactment of the Fugitive Slave Law as part of the Compromise exacerbated sectional tensions.

The question of slavery in the territories exploded once again when Senator Stephen A. Douglas proposed that Kansas and Nebraska territories be opened to white settlement and that the status of slavery be decided according to the principle of popular sovereignty. The Kansas-Nebraska Act convinced many Northerners that the South wanted to open all federal territories to slavery and brought into existence the Republican party, committed to excluding slavery from the territories.

Sectional conflict was intensified by the Supreme Court’s Dred Scott decision, which declared that Congress could not exclude slavery from the western territories and by the abolitionist John Brown’s raid on Harpers Ferry, Virginia.