Settlement of the Great Plains increased rapidly during the 1850s, even while many people moved into Texas and other portions of the territory acquired as a result of the war with Mexico in the 1840s. Congress repeatedly debated whether slavery should be allowed into those large territories that were not yet organized into states. Opponents of slavery opposed plans of slave owners who wished to introduce slavery into the Great Plains and into the territories obtained from Mexico. During the 1850s they grew increasingly suspicious that a conspiracy of slave owners existed that would populate the Great Plains with slavery, create more slave states, and destroy the American system of republican government. On the other hand, many white Southerners feared that restrictions on slavery in western territories would leave too many slaves in the southern states, thus making slavery unprofitable and consequently destroying the region's economy.
In 1854 Congress passed the Kansas-Nebraska Act that provided for the organization of governments in two of the fertile Great Plains territories. The act allowed residents of the territories to decide whether to permit slavery and thereby repealed the Missouri Compromise of 1820 that had forbidden slavery in the northern portion of the Louisiana Purchase. The concept of popular sovereignty that Illinois senator Stephen A. Douglas proposed appeared to be a democratic method of settling disputes about whether slavery should be permitted in the territories. Many opponents of slavery disapproved of popular sovereignty because it could allow slavery into a territory, and many Southern politicians disapproved of popular sovereignty because it could result in territorial legislation that excluded slavery in a territory. Violence between opponents and advocates of slavery erupted in 1856 in Kansas, where more than three dozen settlers were killed, including several supporters of slavery killed by abolitionists under the leadership of John Brown.
In March 1857 the Supreme Court attempted to settle the political dispute about the territories. In the controversial Dred Scott v. Sandford decision, the Court ruled that slaves were not citizens of the United States and that Congress could not prohibit introduction of slavery into the territories. The ruling also appeared to require Congress to protect the property rights of slave owners in the territories. The Court's decision dramatically increased sectional tensions. Disagreements about popular sovereignty intensified within the Democratic Party. Support for the new Republican Party that championed free labor rapidly increased in the free states. Most of the Republican Party's northern and midwestern leaders denounced the Dred Scott decision.