Bleeding Kansas came about as a result of the 1854 Nebraska-Kansas Act. This invalidated the previous Missouri Compromise, which had made all new states admitted to the Union either slave or free, depending on whether they fell above or below the 36°30′ north parallel. Under the Nebraska-Kansas Act, the settlers of an individual territory were allowed to choose whether to enter the Union as free or slave states. This was an outgrowth of the concept of Popular Sovereignty, as championed by Stephen A. Douglas, and sought to defuse the tensions regarding Kansas' entrance to the Union as a free state.

The idea backfired because of two reasons. First, the country was incredibly polarized over the issue of slavery and maintaining a balance of power in Congress. The free states wanted no more slave states to come in, and the slave states wanted no more free states. Second, at the time, Kansas was very sparsely settled. This led to an influx of armed volunteers from the South, mainly Missouri, who sought to influence the decision through their immigration. This was followed by the arrival of New Englanders, who came for the very opposite purpose. It very quickly gave way to internecine violence, which lasted until the end of the American Civil War.

It was quite bloody, as such things go, but certainly not particularly so compared to the war which followed. From 1854-1861, a few dozen people were killed. The violence intensified following the outbreak of the Civil War proper, with the city of Lawrence, Kansas burned, and some 150 abolitionists murdered.

McPherson, James M. Battle Cry of Freedom.