Once the Kansas-Nebraska Act was passed in 1854, many settlers moved into the Kansas Territory. They wanted to decide, through popular sovereignty, if slavery would be allowed there. Many Northerners came hoping to make Kansas a free state. At the same time, many proslavery Missourians entered the territory to sway the elections. Fraud, threats, and violence followed, and Kansas soon became known as “Bleeding Kansas.” Rival governments formed in the territory, each claiming to be the real one. Most settlers wanted peaceful lives, but armed groups on both sides spread fear and destruction.
In May 1856, proslavery forces attacked the antislavery town of Lawrence. Buildings were burned, and property was destroyed. Soon after, the abolitionist John Brown decided to strike back. Brown, who had moved to Kansas with several of his sons, led a small group in an attack at Pottawatomie Creek, where they killed five proslavery men. The killings shocked the territory. Violence in Kansas led to skirmishes, kidnappings, and raids. This included the battles of Black Jack and Osawatomie. These events pushed the territory closer to open warfare and drew the nation's attention.

Brown believed the violence in Kansas showed that peaceful solutions to slavery would fail. Over the next few years, he became convinced that a direct strike against slavery would spark a larger movement. With help from Northern abolitionists known as the Secret Six, Brown planned a raid on Harpers Ferry, Virginia. His goal was to seize weapons from the federal arsenal, free enslaved people in the surrounding area, and begin a rebellion that he hoped would spread across the South.
On the night of October 16, 1859, Brown and 18 of his men carried out the raid on the federal armory at Harpers Ferry. They captured the armory and took hostages as fighting broke out with a local militia. At least four people were killed during the raid, including two civilians, while others died in clashes between Brown’s group, militia members, and later U.S. Marines. By the next day, United States Marines led by Colonel Robert E. Lee had arrived. After a final assault, Brown was wounded and captured. Several of his men were killed, and others were taken prisoner. Brown was tried in Virginia, found guilty of murder, treason, and inciting a slave rebellion, and executed on December 2, 1859.
The aftermath of the raid shook the nation. Many white Southerners felt angry and afraid. They believed Northerners backed Brown’s plan for a slave uprising. They strengthened their militias and no longer trusted Northern politicians who denied involvement. Southern leaders spoke even more fiercely in defense of slavery and treated almost any criticism as a threat. In their view, a radical abolitionist like Brown was not much different from a Republican who wanted to stop slavery's spread.

Northern opinion was divided. Some people condemned Brown’s violence. However, many abolitionists saw him as a martyr who died for freedom. Some meetings in Northern towns honored him. Other Northerners took part in big “Union meetings” to show they opposed the raid. These mixed reactions heightened tensions between the regions. Compromise now seemed almost impossible. The raid convinced many Southerners that peaceful solutions were failing and that their safety depended on breaking from the Union. Less than two years after Brown's raid, the United States would enter the Civil War.