Bleeding KansasBloody Kansas, or the Border War was a series of violent civil confrontations in the Kansas Territory between 1854 and 1861. These emerged from a political and ideological conflict over the legality of slavery in the proposed state of Kansas. The conflict was characterized by years of electoral fraud, raids, assaults, and retributive murders carried out in Kansas and neighboring Missouri by pro-slavery "Border Ruffians" and anti-slavery "Free-Staters", John Brown in particular.

There were during part of this period simultaneously two different governments, in different cities, with different constitutions, each — one slave, one free — claiming to be the legitimate government of the entire Kansas Territory.

At the core of the conflict was the question of whether the territory would allow or prohibit slavery, and thus enter the Union as a slave state or a free state. The Kansas–Nebraska Act of 1854 called for popular sovereignty, specifying that the decision about slavery would be made by popular vote of the territory's settlers, rather than legislators in Washington. This apparently rational solution did not work because there was no way to determine whether a man desiring to vote was a resident of Kansas or not. This question became a bitterly disputed matter.

The only state with which Kansas Territory had a border was the slave state of Missouri. It was easy for Missouri slaveowners, and other slavery sympathizers, to cross the border into Kansas and set up a homestead, genuinely or fraudulently, bringing their enslaved persons with them, if they owned any. Anti-slavery societies in the Northeast sponsored moves to the territory of prospective homesteaders who agreed to oppose slavery. The most famous of these was John Brown.

Those in favor of slavery argued that every settler had the right to bring his own property, including slaves, into the territory. In contrast, while some "Free Soil" proponents opposed slavery on religious, ethical, or humanitarian grounds, at the time the most persuasive argument against introducing slavery in Kansas was that it would allow rich slaveholders to control the land, to the exclusion of white non-slaveholders who regardless of their moral inclinations did not have the means to acquire either slaves or sizable land holdings for themselves.

Missouri, a slave state since 1821, was populated by settlers with Southern sympathies and pro-slavery views, some of whom tried to influence the decision by entering Kansas and claiming to be residents. This faction used brutal gang violence and paramilitary guerrilla warfare, although there was also some among the anti-slavery activists. (See Pottawatomie massacre.) At the same time, anti-slavery societies in the Northeast were helping anti-slavery settlers move to Kansas.

Kansas was admitted to the Union as a free state on January 29, 1861, which was made possible by the departure of the legislators from the states that had seceded earlier in January. (A long-standing, similarly contentious issue whose resolution also became possible was ending slavery in the District of Columbia.) While partisan violence continued along the Kansas–Missouri border for most of the war, Union control of Kansas was never seriously threatened.

The term "Bleeding Kansas" was popularized by Horace Greeley's New York Tribune. Bleeding Kansas demonstrated the gravity of the era's most pressing social issues, from the matter of slavery to states' rights. Its severity made national headlines, which suggested to the American people that the sectional disputes were unlikely to be resolved without bloodshed, and it therefore anticipates the American Civil War.

The episode is commemorated with numerous memorials and historic sites.

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