Answer:
The Kansas-Nebraska Act, that allowed people from the territories of Kansas and Nebraska to decide, through popular sovereignty, whether becoming a free or slave state, led to a pre-Civil War clash between abolitionists and pro-slavery activists in Kansas, as both groups wanted to establish their respective views regarding slavery in the territory. This event was called Bleeding Kansas, and had an important impact on the political situation in the United States at the time and was one of the leading events of the subsequent Civil War.
The conflict developed from 1854 to 1861, with estimated casualties of more than 100 deaths between both sides. Finally, Kansas was admitted to the Union as a free state, although pro-slavery activists established a sort of parallel government in the state during the Civil War.