In January 1854, Senator Stephen Douglas introduced a bill that divided the land west of Missouri into two territories, Kansas and Nebraska. He argued for popular sovereignty, which would allow the settlers of the new territories to decide if slavery would be legal there. Antislavery supporters were outraged because, under the terms of the Missouri Compromise of 1820, slavery would have been outlawed in both territories.

After months of debate, the Kansas-Nebraska Act passed on May 30, 1854. Pro-slavery and anti-slavery settlers rushed to Kansas, each side hoping to determine the results of the first election held after the law went into effect. The conflict turned violent, aggravating the split between North and South until reconciliation was virtually impossible.

Opponents of the Kansas-Nebraska Act helped found the Republican Party, which opposed the spread of slavery into the territories. As a result of the Kansas-Nebraska Act, the United States moved closer to Civil War.