He had been working on this plan since at least 1847, when he had first shared it with the noted Maryland-born former slave and Abolitionist leader, Frederick Douglass. The plan called for striking at the very heart of the slave South by launching a guerrilla war in the Appalachian mountains.
The federal census returns from 1850 were analyzed county by county to establish black populations, free and slave, throughout the South. Maps were carefully studied and annotated, and financial backers and recruits were enlisted throughout New England, the West, and Canada.
Brown selected Harper’s Ferry, Virginia, as the first point of attack in his projected war of slave liberation. Situated on the Potomac River, Harper’s Ferry seemed to offer everything Brown could hope for. It was in Virginia, the largest slaveholding state in the Union. It was on the edge of the Appalachian Mountains. Harpers Ferry had the federal arsenal and the large gun factory that produced rifles for the U.S. government.
Once in control of Harper’s Ferry, the guns from the federal armory and the gun factory, as well as pikes, would be distributed to the slaves and sympathetic whites whom Brown was certain would flock to him in support of his cause. Once the new recruits were armed, the revolution would be carried further south along the Appalachian chain, with points of attack concentrating on other towns with federal arsenals, until the army of slave liberators reached the Gulf.
In Chatham, Canada, a convention was called, where a constitution was adopted for the proposed new country for freed slaves. It would be located in the southern Appalachian Mountains, and taken by force by the army of the provisional government.
Brown was confident that people held in bondage would join his guerrilla force. John Brown also elected by the delegates to the Chatham Convention as commander-in-chief of the provisional forces, since Brown was the mastermind of the plan and architect of the constitution and projected state.
Brown’s plan got off to an auspicious start, as he and eighteen followers left their rented Maryland farm, some four and one-half miles from Harper’s Ferry, on the evening of Sunday, October 16. Shortly after 10:30 that night they crossed the Potomac River bridge into the town, and quickly overpowered the lone man guarding the gates to the federal arsenal. The rest of the night was spent rounding up prominent men from the town and surrounding countryside to hold as hostages. By daybreak, Brown and his men had secured themselves in the small but sturdy brick engine house on the grounds of the federal arsenal.
The first shot mortally wounded Heyward Shepherd. Shepherd was a free black man who was a night baggage porter for the B&O Railroad that ran through Harpers Ferry near the armory.
The noise from that shot roused Dr. John Starry from his sleep shortly after 1:00 a.m. He walked from his nearby home to investigate the shooting and was confronted by Brown's men. Starry stated that he was a doctor but could do nothing more for Shepherd, and Brown's men allowed him to leave. Instead of going home, Starry went to the livery and rode to neighboring towns and villages, alerting residents to the raid.
When he reached nearby Charles Town, they rang the church bells and aroused the citizens from their sleep. John Brown's men were quickly pinned down by local citizens and militia, and forced to take refuge in the engine house adjacent to the armory.
The Secretary of War asked for the assistance of the Navy Department for a unit of United States Marines, the nearest troops.  Lieutenant Israel Greene was ordered to take a force of 86 Marines to the town. In need of an officer to lead the expedition, U.S. Army Lieutenant Colonel Robert E. Lee was found on leave nearby and was assigned as commander along with Lieutenant J. E. B. Stuart as his aide-de-camp.
The whole contingent arrived by train on October 18, and after negotiation failed, they stormed the fire house and captured most of the raiders, killing a few and suffering a single casualty themselves.
Brown was tried for treason against the state of Virginia, convicted, and hanged in nearby Charles Town. Starry's testimony was integral to his conviction. Following the prosecution (by Andrew Hunter), "John Brown captured the attention of the nation like no other abolitionist or slave owner before or since." The Marines returned to their barracks and Colonel Lee returned to finish his leave. The raid was a catalyst for the Civil War.
1. Sullivan, David (1997). The United States Marine Corps in the Civil War - The First Year. White Mane Publishing Company, Inc.. pp. 1 to 27.