In January 1854, Senator Stephen A. Douglas of Illinois proposed a bill to organize Nebraska (part of the Louisiana Purchase) as a territory, in order to facilitate the building of a transcontinental railroad along a northern route from Chicago to the West. Because the Nebraska Territory lay above the 36º30' line, set by the Missouri Compromise to disallow slavery, Nebraska would automatically become a candidate for admission as a free state. Southerners therefore planned to oppose the bill unless Douglas made some concessions.

To ensure passage of the bill, Douglas yielded to Southerners who desired to void the Missouri Compromise’s 36º30' line. He inserted in his Nebraska bill an explicit repeal of the Missouri Compromise so that no territory would be automatically designated non-slaveholding. As an alternative, the bill declared that the slavery issue in the Nebraska region would be decided by popular sovereignty, thus extending the Compromise of 1850’s concept of popular sovereignty to territories outside New Mexico and Utah. Douglas further divided the Nebraska Territory into two parts: Nebraska to the west of Iowa, and Kansas to the west of Missouri. Many assumed that this meant Kansas would be reserved for slavery and Nebraska for free soil. With these concessions attached, the bill passed through Congress and became law in May 1854.

The Kansas-Nebraska Act, however, did not stave off sectional conflict. Because Nebraska was likely to prohibit slavery, as a territory above the 36º30' line, Kansas became a battleground for sectional interests. Both Northern abolitionist groups and Southern interests rushed into the territory to try to control the local elections. In March 1855, during the first election of the territorial legislature, thousands of pro-slavery inhabitants of Western Missouri crossed into Kansas to tilt the vote in favor of slaveholding interests. Because of the election fraud perpetrated by these “border ruffians,” a pro-slavery government swept into power. This new government immediately ousted antislavery legislators and set up a pro-slavery constitution known as the Lecompton Constitution.

In opposition to the new legislature, abolitionist John Brown led a massacre of five men at a pro-slavery camp, setting off an outbreak of violence. More than 200 people died in the ensuing months of violence, earning the territory the nickname Bleeding Kansas. Three years later, in 1859, Brown led an even larger antislavery revolt in Virginia, when he attempted to seize federal arsenal at Harpers Ferry in order to arm a massive slave uprising. His raid was unsuccessful, however, and he was caught and hanged.

Distraught by the violence of Bleeding Kansas, President James Buchanan, who was elected in 1856, sought a judicial resolution to the issue of slavery’s extension. A case he saw as potentially providing such a resolution was that of Dred Scott, in which Scott, a Missouri slave, sued for his freedom on the basis that his owner had taken him to live in a free state (Illinois), and later a free territory (Wisconsin). In March 1857, Chief Justice Roger B. Taney delivered the majority opinion on Dred Scott v. Sandford.

To begin his ruling, Taney stated that Scott, as a slave, had no right to sue in federal court, and further claimed that no black, whether slave or free, could become a citizen of the United States. Slaves were property only, according to Taney, and would remain property even if they resided in free territory. Furthermore, Taney ruled that Congress could not forbid slavery in any U.S. territory because doing so would violate the Fifth Amendment’s protection of property, including slaves, from being taken away without due process. This decision rendered the Missouri Compromise unconstitutional (though the Compromise had already been effectively nullified by the Kansas-Nebraska Act). Taney further suggested that the Compromise of 1850 and the Kansas-Nebraska Act were unconstitutional, since they enforced popular sovereignty, which allowed territorial governments to prohibit slavery and therefore violated the Fifth Amendment as interpreted by the Court. Though Buchanan initially had hoped that the Dred Scott ruling might resolve the debates about extending slavery, it actually aggravated sectional tensions. Northerners harshly condemned the ruling, while Southerners celebrated it.