The Kansas-Nebraska Act was a significant piece of legislation passed by the U.S. Congress in 1854. It created the territories of Kansas and Nebraska and allowed the settlers in those territories to determine through popular sovereignty whether they would allow slavery within their borders. This act effectively repealed the Missouri Compromise of 1820, which had prohibited slavery in that part of the Louisiana Purchase north of latitude 36°30′.
The act was introduced by Senator Stephen A. Douglas, who sought to promote westward expansion and the construction of a transcontinental railroad. However, its passage led to intense conflict between pro-slavery and anti-slavery factions, particularly in Kansas, resulting in violent confrontations known as "Bleeding Kansas." The act heightened tensions leading up to the Civil War and contributed to the fracturing of existing political parties, notably leading to the rise of the Republican Party, which was founded in opposition to the expansion of slavery.
Overall, the Kansas-Nebraska Act played a crucial role in the events leading up to the American Civil War, illustrating the deep divisions over the issue of slavery in the United States.