Bleeding Kansas, Bloody Kansas or the Border War, was a series of violent political confrontations involving anti-slavery Free-Staters and pro-slavery "Border Ruffian" elements, that took place in the Kansas Territory and the neighboring towns of Missouri between 1854 and 1861. At the heart of the conflict was the question of whether Kansas would enter the Union as a free state or slave state. As such, Bleeding Kansas was a proxy war between Northerners and Southerners over the issue of slavery in the United States. The term "Bleeding Kansas" was coined by Horace Greeley of the New York Tribune; the events it encompasses directly presaged the American Civil War.

Congress had long struggled to balance the interests of pro-slavery forces and anti-slavery forces. The events later known as Bleeding Kansas were set into motion by the Kansas–Nebraska Act of 1854, which nullified the Missouri Compromise and instead implemented the concept of popular sovereignty. An ostensibly democratic idea, popular sovereignty stated that the inhabitants of each territory or state should decide whether it would be a free or slave state; however, this resulted in immigration en masse to Kansas by activists from both sides. At one point, Kansas had two separate governments, each with its own constitution, although only one was federally recognized. On January 29, 1861, Kansas was admitted to the Union as a free state, less than three months before the Battle of Fort Sumter which began the Civil War.