 In 1854, the Kansas–Nebraska Act nullified the policy set by the Missouri Compromise by permitting the Kansas and Nebraska Territories to vote on whether they would join the Union as free or slave states.The result was a "de facto" war between pro-slavery residents of Missouri, called "Border Ruffians", and anti-slavery "Free-Staters" of Kansas, each of which sought to influence how Kansas entered the Union.The conflict involved attacks and murders of supporters on both sides, with the Sacking of Lawrence by pro-slavery forces and the Pottawatomie massacre led by abolitionist John Brown the most notable.