At the outbreak of the American Civil War in April 1861, Kansas was the newest U.S. state, admitted just months earlier in January. The state had formally rejected slavery by popular vote and vowed to fight on the side of the Union, though ideological divisions with neighboring Missouri, a slave state, had led to violent conflict in previous years and persisted for the duration of the war.
While Kansas was a rural frontier state distant from the major theaters of war and its Unionist government was never seriously threatened by Confederate military forces, several engagements did occur within its borders, as well as countless raids and skirmishes between local irregulars, including the Lawrence Massacre by pro-Confederate guerrillas under William Quantrill in August 1863. Later the state witnessed the defeat of Confederate General Sterling Price by Union General Alfred Pleasonton at the Battle of Mine Creek, the second-largest cavalry action of the war.
The decision of how Kansas would enter the Union was a pivotal one which forced the entire country to confront the political and social turmoil generated by the question of abolition and contributed to the strong division in sentiment that eventually erupted into war. The early violence there presaged the coming national conflict, and throughout the war Kansas remained a staunchly loyal Union stronghold at the western edge of a border region otherwise populated by uneven governments and mixed sympathies.
The Territory of Kansas was admitted to the Union on January 29, 1861, in the midst of the national secession crisis: six states had already seceded, and five more would follow in the coming months. The Kansas–Nebraska Act of 1854 had rescinded the former Missouri Compromise and permitted the territories of Kansas and Nebraska to determine whether they would enter the Union as slave or free states by popular sovereignty. Violence between pro-slavery and anti-slavery groups began almost immediately. The conflict was especially bloody along the Kansas–Missouri border, where Missouri Border Ruffians and Kansas Free-Staters formed bands of partisan rangers to raid and pillage opposition strongholds, earning it the name "Bleeding Kansas". Kansas' popular vote eventually chose against slavery, so Kansas would fight with the North.