Answer:
The Kansas-Nebraska Act, issued on May 30, 1854 created the territories of Kansas and Nebraska, repealing the Missouri Compromise, and allowing immigrants settled in these territories to decide whether or not to introduce slavery on them.
The text stated that the pioneers would be able to vote to decide whether or not to introduce slavery, in the name of "popular sovereignty". Unsurprisingly, opponents of slavery denounced the law, viewing it as a concession to the slave power of the South. The new Republican Party, which was created in opposition to this law, set itself the goal of stopping the expansion of slavery and quickly became the dominant force in all the northern states.
The result was a series of violence and murders called Bleeding Kansas between 1854 and 1861, pitting pro and anti-slavery settlers in the new Kansas Territory, and revealing itself as the origin of the Civil War.