Marais des Cygnes massacre
The Marais des Cygnes massacre (, also ) is considered the last significant act of violence in Bleeding Kansas prior to the outbreak of the American Civil War. On May 19, 1858, approximately 30 border ruffians led by Charles Hamilton, a Georgia native and proslavery leader, crossed into the Kansas Territory from Missouri.
Source: Wikipedia — Marais des Cygnes massacre (CC BY-SA 4.0)