Kansas-Nebraska Act

Kansas-Nebraska Act (1854), United States law authorizing the creation of Kansas and Nebraska, west of the states of Missouri and Iowa and divided by the 40th parallel. It repealed a provision of the Missouri Compromise (1820) that had prohibited slavery in the territories north of 36° 30', and stipulated that the inhabitants of the territories should decide for themselves the legality of slaveholding. The passage of the act caused a realignment of the major US political parties and greatly increased tension between North and South in the years before the American Civil War.

The Kansas-Nebraska Bill was sponsored by the Democratic senator Stephen A. Douglas of Illinois. By opening up what had been Native American country to white settlement, Douglas and other northern leaders hoped to facilitate construction of a transcontinental railway through their states rather than through the southern part of the country. The removal of the restriction on the expansion of slavery ensured southern support for the bill, which was signed into law by President Franklin Pierce on May 30, 1854.

The act's passage and stormy implementation split the Democratic party and destroyed the already badly divided Whig party. Whig opposition to the measure practically ended support for that party in the South. The northern Whigs joined antislavery Democrats and Know-Nothings to form the Republican party in July 1854.

A conflict soon developed in Kansas between proslavery settlers from Missouri and antislavery newcomers who began to move into the territory from the north-eastern states, such as John Brown. Fighting between the two groups, known as the Border War, continued for several years, aggravating the sectional controversy that led to the outbreak of the Civil War in 1861.