The Kansas-Nebraska Act was a U.S bill organising U.S. territory for settlement preparatory for its later admission to the Union as the states of Kansas and Nebraska. Prior to the act's passage the territory was called the Nebraska Territory. Its passage, in 1854, exacerbated the progressive polarisation of the U.S. over the issue of slavery and thereby formed part of build up to the American Civil War.

The bill was proposed by Stephen A. Douglas of Illinois and passed by the U.S. Congress on May 30, 1854 after fierce debate. It created the separate territories of Kansas and Nebraska allowing each to decide whether to allow slavery within their borders (this was an expression of the doctrine of popular sovereignty). The Act repealed the Missouri Compromise of 1820 which prohibited slavery north of latitude 36°30´, though the compromise itself was later held to be unconstitutional by the Supreme Court in the Dred Scott case.

At the time of passage of the act, slavery supporters were somewhat more numerous than their opponents among the settlers in Kansas. There was no significant support for the institution of slavery in Nebraska. After the Kansas-Nebraska Act was passed, both pro- and anti-slavery supporters attempted to muster settlers of their own persuasion to settle in Kansas. The anti-slavery New England Emigrant Aid Company, headed by Amos Lawrence, was highly successful in this project, and a nucleus of anti-slavery sentiment was established about the town of Lawrence, Kansas.

Pro-slavery settlers migrated to Kansas mainly from Missouri. Their influence in territorial elections was often bolstered by resident Missourians who crossed the border into Kansas purely for the purpose of voting in such ballots. These interlopers were called border ruffians by their opponents, a term coined by Horace Greeley. The territorial capital of Lecompton, Kansas was the target of this agitation and consequently became such a hostile environment for Free-Soilers, that they set up their own unofficial legistlature at Topeka.

The hostilities between the factions reached a state of low intensity civil war which was severely embarrassing to the Federal Government, especially as the nascent Republican Party sought to capitalise on the scandal of Bleeding Kansas. Successive territorial governors attempted to maintain the peace. They were usually sympathetic to slavery, but found themselves unable to countenance the routine ballot rigging and intimidation that was practiced far more intensively by pro-slavery settlers as they lost the race to populate the territory.

The pro-slavery territorial legislature ultimately proposed a state constitution for approval by referendum. The constitution was offered in two alternative forms, neither of which unambiguously made slavery illegal. Free-Soil settlers boycotted the legislature's referendum and organised their own which approved a free state constitution. The results of the competing referendums were sent to Washington D.C by the territorial governor.

President James Buchanan sent the Lemcompton constitution to Congress for approval and proclaimed Kansas to be a state. The Senate approved the admission of Kansas as a state under the Lecompton constitution, but this measure was blocked in the House, with the opponents of Kansas' admission as a slave state including Stephen A. Douglas. Senator James Hammond of South Carolina (famous for his King Cotton speech) characterised this resolution as the expulsion of the state, asking If Kansas is driven out of the Union for being a slave state, can any Southern state remain within it with honour?

Eventually a new anti-slavery constitution was drawn up. On January 29, 1861, Kansas was admitted to the Union as a free state. Nebraska was admitted to the Union as a state after the Civil War in 1867.