From 1856 to 1860, sectional tensions between the North and South intensified due to several key events and issues: 1) The passage of the Kansas-Nebraska Act in 1854 established the principle of "popular sovereignty" but led to violent clashes in "Bleeding Kansas" as both sides fought over whether Kansas would be a slave or free state. 2) The Dred Scott decision in 1857 inflamed tensions by ruling that Congress could not regulate slavery in the territories and that blacks were not citizens. 3) John Brown's raid on Harper's Ferry in 1859 and the publication of Hinton Helper's book Impending Crisis of the South increased Southern fears of Northern abolitionism. 4