Bleeding Kansas is a term coined by newspaper editor Horace Greeley to describe the bitter clash between anti-slavery and pro-slavery factions. Both sides fought and spilled blood over Kansas joining the Union as a free or slave state.
In 1854, the citizens of the Kansas territory were given the right to determine if Kansas would be a free or slave state. The Kansas-Nebraska Act used the concept of popular sovereignty to overrule the Missouri Compromise. People on both sides of the slavery debate rushed into Kansas to influence the decision.
Ruffians from the Missouri border crossed into Kansas and attacked abolitionists and free state supporters. Some abolitionists, like John Brown, took up arms to fight back. People were killed during these clashes, which led to Horace Greeley coming up with his famous Bleeding Kansas phrase.
The political and physical battles in Kansas went on for several years. Both sides had formed a hard line and refused to compromise. Free-state settlers continued to flow into Kansas from northern states. The town of Lawrence, Kansas was looted by pro-slavery ruffians, which sparked abolitionist John Brown to respond with violence of his own.
Eventually, the citizens of Kansas voted to become a free state in 1861.