Two pressing issues occupying American politics prior to the Civil War converge in the Kansas-Nebraska Act: slavery and westward expansion.
Settlers seeking to stake a claim in what would become Nebraska were prevented from doing so because it was not a territory. The Missouri Compromise prevented slavery north of the line running along the southern border of Missouri. Southerners did not want a new state north of the line to upset the balance of powers regarding slave states. Douglas proposed the Kansas-Nebraska Act, which allowed people in these two territories to decide for themselves if they wanted to allow slavery. This undermined the longer-standing Missouri Compromise, and likely proved one of the more decisive events leading to the Civil War.
The abolitionist John Brown became involved in the violence that ensued as a result of this act, in what is known as Bleeding Kansas. While Nebraska was securely an anti-slavery state, Douglas assumed Kansas would be pro-slavery, keeping the number of Northern and Southern states the same as before the act. However, he underestimated the resistance to slavery some in Kansas would have and the level of violence that would result.
This acrimony eventually led not only to the Civil War but also to the destruction of the Whig Party. After this act, Northern and Southern Whigs split form each other, with Southern Whigs joining the Democratic Party and the Northern Whigs forming the Republican Party, soon to be associated with Abraham Lincoln.