The Kansas-Nebraska Act of 1854 represented a dramatic change in the mechanisms designed to maintain the balance of power between free and slave states in 19th century American government. The Missouri Compromise had seemingly settled the issue in 1820 by declaring that all states formed from lands acquired in the Louisiana Purchase lying south of 36°30' N would allow slavery, while those north would not. The exception was to be Missouri itself. After this legislation was enacted, states were admitted to the Union in pairs of one northern and one southern state in order to maintain an equal number of free and slave states.

The Kansas-Nebraska Act (1854) effectively shattered this uneasy truce. Introduced by Senator Stephen A. Douglas (D-IL), this act had two major effects: it divided Nebraska Territory into two states (Kansas and Nebraska) and declared that these new states would decide whether or not to allow slavery by means of a popular vote when they joined the Union. Known as popular sovereignty to its supporters and squatter sovereignty to its detractors, this legislation made it possible for free states to outnumber slave states, or vice versa, in the American government, thus upsetting the delicate balance of power that had been established in 1820 and triggering a major realignment of the leading political parties. It also led to a violent reopening of the slavery question among the people living in the territories in a series of events now known as Bleeding Kansas.