The Kansas-Nebraska Act was a law passed by the U.S. Congress in 1854. It created the territories of Kansas and Nebraska, allowing the settlers in those territories to decide whether to allow slavery through the principle of popular sovereignty. This act effectively repealed the Missouri Compromise, which had previously prohibited slavery in that region. The act led to significant conflict and violence, known as Bleeding Kansas, as pro-slavery and anti-slavery settlers rushed into the territories to influence the decision. This turmoil highlighted the deep divisions in the United States over the issue of slavery, contributing to the tensions that eventually led to the Civil War.