The Marais des Cygnes Massacre 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 men led by Charles Hamilton, a Georgian native and proslavery leader, crossed into Kansas from Missouri. They arrived at Trading Post, Kansas in the morning and then headed back to Missouri. Along the way they captured 11 free-state men, none of whom was armed and, it is said, none of whom had participated in the ongoing violence. Most of the men knew Hamilton and apparently did not realize he meant them harm. These prisoners were led into a defile, where Hamilton ordered the men shot and fired the first bullet himself. Five men were killed.
Hamilton and his gang returned to Missouri. Only one man was ever brought to justice. William Griffith of Bates County, Missouri, was arrested in the spring of 1863 and hanged on October 30 of that year. Charles Hamilton returned to Georgia, where he died in 1880.
The incident horrified the nation, and inspired John Greenleaf Whittier to write a poem on the murder, "Le Marais du Cygne," which appeared in the September 1858 Atlantic Monthly. 
John Brown arrived at the scene toward the end of June and built a "fort" some 220 yards south of the ravine where the men had been shot, on land belonging to a local blacksmith named Eli Snider. It was reported to have been two stories high, walled up with logs and with a flat roof. Water from a spring ran through the house and into a pit at the southwest corner. The land was later sold to Brown's friend Charles C. Hadsall, who agreed to let Brown occupy it for military purposes. Brown and his men withdrew at the end of the summer, leaving the fort to Hadsall.
Hadsall later built a stone house adjoining the site of Brown's fort, enclosing the spring within the walls of the first floor. In 1941 the Kansas legislature authorized acceptance of the massacre site, including Hadsall's house, as a gift to the state from the Pleasanton Post, Veterans of Foreign Wars. In 1963 it provided funds for the restoration of the building, and in 1969 the entire property was turned over to the Kansas Historical Society for administration as the Marais des Cygnes Massacre State Historic Site. A museum was established in the upper floor of the building in 1964.
la:Internecio Paludis Cygnorum