Border Ruffians were pro-slavery activists from Missouri who crossed into Kansas in the mid-1850s to influence the decision of whether Kansas would enter the Union as a free or slave state. These individuals played a significant role during the violent conflicts known as 'Bleeding Kansas,' which arose from the tensions between pro-slavery and anti-slavery factions. Their actions were a manifestation of the broader national struggle over slavery and contributed to the breakdown of compromise between North and South.