John Brown's actions, first in the conflict known as "Bleeding Kansas" and later his failed attempt at leading a slave uprising in Virginia, brought him national notoriety and increased tensions between the North and South before the 1860 election.
Brown had been an active and committed abolitionist for much of his life before he moved to Kansas in 1855. Kansas, having just been organized under the Kansas-Nebraska Act of 1854, was in the midst of what amounted to a civil war as pro-slavery and anti-slavery settlers fought to determine whether or not the institution would be allowed there. In retribution for the destruction of the anti-slavery town of Lawrence, Brown and his sons hacked five pro-slavery men to death at Pottawattomie Creek. This action made him infamous in the South, but earned him the admiration of some abolitionists, incensed over the turn events had taken in Kansas. Brown was able to parlay this fame into funding and support for a scheme to launch a slave uprising in the South.
In October of 1859, Brown led a force of twenty-six men, including three of his sons and several free blacks, in an attack on the federal arsenal at Harpers Ferry, Virginia. He hoped to capture the arsenal and use the weapons there to arm slaves he thought would rise up when they heard the news of his raid. The arsenal, however, was quickly surrounded by townspeople and soon after federal forces, and Brown was arrested. He was hanged just over a month later. Brown's death made him a martyr for some in the North, and this reaction, coupled with the fact that some prominent abolitionists had funded the venture shocked and horrified the South.