This chapter discusses Mazzini's influence in the context of the slavery crisis of the 1850s in the United States. That decade, which saw a crisis erupt in Kansas over the question of whether slavery should be allowed to expand, ended dramatically at Harpers Ferry, Virginia, where the violent abolitionist John Brown led a doomed attempt to arm and liberate slaves. Mazzini studied, wrote about, and on occasion attempted to enact popular insurrection and guerilla warfare. His ideas became essential to Brown's ideology and actions, which precipitated the Civil War. The chapter suggests an under-appreciated aspect of Mazzini's influence in America, invites a reassessment of the American sectional crisis of the 1850s for its transatlantic dimensions, and proposes a sobering but important dimension to the historical path of the spread of democratic nationalism.