Kansas-Nebraska Act (1854)
This bill, introduced by Stephen Douglas, divided the Nebraska Territory into two parts: Kansas and Nebraska. The Act also overturned the Missouri Compromise, which provided that, except for Missouri itself, territories above 36°30’ north latitude would be free states. The Kansas-Nebraska Act left it up to state legislatures to decide the slavery question for their respective states – a concept referred to as "popular sovereignty."
Years later in 1865, the question of slavery would be settled once and for all with the ratification of the Thirteenth Amendment.