The passing of the Kansas-Nebraska Act in 1854 was the single most important event in the inception of the era of “ Bleeding Kansas . ” Whether the new Kansas Territory would be slave or free was left up to popular sovereignty , counter to the prohibitions of the Missouri Compromise . An immediate rush of migrants on both sides of the issue rushed in to settle and to determine the fate of the new territory . Almost immediately , violent struggles erupted into a full-blown border war between the “ Free-Staters ” and the “ Border Ruffians . ” Following the 1855 arrival of the virulent abolitionist John Brown in Kansas , open conflict escalated with the Border Ruffians ’ May 21 , 1856 sacking of the Free-Stater town Lawrence . In retaliation , four days later , Brown and his followers committed what became known as the Pottawatomie massacre against five pro-slavery men . The Battle of Black Jack followed soon after in early June , ending in the anti-slavery forces ’ favor and making Brown a threat in the eyes of the Border Ruffians . Violence in Kansas steadily increased thereafter , throughout the summer
Pottawatomie massacre
Pottawatomie massacre
Battle of Osawatomie