The Pottawatomie massacre occurred on the night of May 2425, 1856, in the Kansas Territory. In reaction to the sacking of Lawrence by pro-slavery forces on May 21, and the telegraphed news of the severe attack on May 22 on Massachusetts Senator Charles Sumner, for speaking out against slavery in Kansas ("The Crime Against Kansas"), John Brown and a band of abolitionist settlerssome of them members of the Pottawatomie Riflesmade a violent reply. Just north of Pottawatomie Creek, in Franklin County, they killed five pro-slavery settlers, in front of their families. This soon became the most famous of the many violent episodes of the "Bleeding Kansas" period, during which a state-level civil war in the Kansas Territory was described as a "tragic prelude" to the American Civil War which soon followed. "Bleeding Kansas" involved conflicts between pro- and anti-slavery settlers over whether the Kansas Territory would enter the Union as a slave state or a free state. It is also John Brown's most questionable act, both to his friends and his enemies. In the words of abolitionist Frederick Douglass, it was "a terrible remedy for a terrible malady.":371

John Brown (May 9, 1800 – December 2, 1859) was an American abolitionist leader. First reaching national prominence for his radical abolitionism and fighting in Bleeding Kansas, he was eventually captured and executed for a failed incitement of a slave rebellion at Harpers Ferry preceding the American Civil War. A man of strong religious convictions, Brown believed he was "an instrument of God",: 248  raised up to strike the death blow to American slavery, a "sacred obligation".: 189  Brown was the leading exponent of violence in the American abolitionist movement:: 426  he believed that violence was necessary to end American slavery, since decades of peaceful efforts had failed. Brown said repeatedly that in working to free the enslaved he was following the Golden Rule, as well as the U.S. Declaration of Independence, which states that "all men are created equal".: 721  He also stated often that in his view, these two principles "meant the same thing".Brown first gained national attention when he led anti-slavery volunteers and his own sons during the Bleeding Kansas crisis of the late 1850s, a state-level civil war over whether Kansas would enter the Union as a slave state or a free state. He was dissatisfied with abolitionist pacifism, saying of pacifists, "These men are all talk. What we need is action – action!". In May 1856, Brown and his sons killed five supporters of slavery in the Pottawatomie massacre, a response to the sacking of Lawrence by pro-slavery forces. Brown then commanded anti-slavery forces at the Battle of Black Jack and the Battle of Osawatomie.

In October 1859, Brown led a raid on the federal armory at Harpers Ferry, Virginia (today West Virginia), intending to start a slave liberation movement that would spread south; he had prepared a Provisional Constitution for the revised, slavery-free United States he hoped to bring about. He seized the armory, but seven people were killed, and ten or more were injured. Brown intended to arm slaves with weapons from the armory, but only a few slaves joined his revolt. Those of Brown's men who had not fled were killed or captured by local militia and U.S. Marines, the latter led by Robert E. Lee. Brown was tried for treason against the Commonwealth of Virginia, the murder of five men, and inciting a slave insurrection. He was found guilty of all counts and was hanged on December 2, 1859, the first person executed for treason in the history of the United States.: 179 The Harpers Ferry raid and Brown's trial, both covered extensively in national newspapers, escalated tensions that led, a year later, to the South's long-threatened secession and the American Civil War. Southerners feared that others would soon follow in Brown's footsteps, encouraging and arming slave rebellions. He was a hero and icon in the North. Union soldiers marched to the new song "John Brown's Body", that portrayed him as a heroic martyr. Brown has been variously described as a heroic martyr and visionary, and as a madman and terrorist.