The Kansas-Nebraska Act effectively nullified the Missouri Compromise by allowing for popular sovereignty (the idea that the residents of a territory should decide for themselves whether or not to allow slavery) in the new territories of Kansas and Nebraska. This meant that the precedent set by the Missouri Compromise, which had prohibited slavery in the northern territories, was no longer in effect. This led to increased tensions between pro-slavery and anti-slavery factions, ultimately contributing to the outbreak of the Civil War.
