Twenty-one men is not an auspicious force with which to seize a federal arsenal and hold it against all comers, to be sure. Nor was John Brown possessed of a particularly talented military mind. (His son John, Jr. was probably better at least insofar as he actually had commanded something like a company of militia in Kansas.) But Brown had some reasons for going ahead that break down to his expecting reinforcements that did not arrive.

Brown knew, from extensive personal experience, that quite a lot of northern whites disliked enslaving. Many had records of supporting at least financial efforts to keep the freedom of enslaved people who stole themselves into the North and those who put their own freedom on the line to help such people were held in some esteem. Though those opinions are far from universal, helping a fugitive or alleged fugitive enslaved person escape kidnapping was reasonably popular for decades. In the 1850s this had included high-profile and sometimes violent fugitive rescues. It wasn't out of the question that antislavery whites in the North might come to his aid with more than money. They didn't show, despite a lot of letter writing and John, Jr. taking a tour of New York, Massachusetts, and Canada. In the case of that tour, Brown's instructions to his son weren't especially specific as to what was wanted and Junior was probably also not in an ideal mental state to go about recruiting, to the point that he seems to have told potential allies conflicting stories about what the idea was. For a while even the finances were in doubt, though Brown's allies were more forthcoming with cash.

Northern Blacks may have offered more hope. Brown specifically wanted prominent Black Americans like Harriet Tubman and Frederick Douglass to join up, or at least use their influence to get him people who would. Douglass agreed to a meeting with Brown over August 19-21 and got this pitch:

Come with me, Douglass. I will defend you with my life. I want you for a special purpose. When I strike, the bees will begin to swarm, and I shall want you to help hive them.

Douglass' turning him down was a substantial blow, among all the others, and Brown evinces some genuine despair about the mission's future. He thought maybe the Almighty had ordained another hour, or that he had misjudged his own destiny. This resulted in a bunch of worship services where he hoped to get his recruits right with God and feared that his and their "impudence and folly" would bring it all to ruin. But at the same time he wrote Junior that the hour had come.

Except maybe just not yet. Brown and his men rented a farm in Maryland and stayed there for some time, the men becoming increasingly restless. Brown wrote out a past-tense defense of the attack, assuming he tried and failed. His recruits worried that the neighbors suspected something and a lynch mob might come for them. Brown made an emergency trip to Philadelphia to find more people among the city's free black population. They did not materialize, but late in September he did get a lone recruit, Osborn P. Anderson, down from Canada and a shipment of pikes and rifles.

It wasn't nothing. Around the 30th Brown decided for sure to go. He sent away two teenage relations he'd had come down to provide cover for the rented farm, his son Oliver's wife (17) and his own daughter Anne (15) and wrote out a farewell letter to his wife and daughters. He sent one of his men to get census data about how many people were locally enslaved...but made no effort to inform them that help was on the way or to scout out trails he might use to escape. Maybe that goes down to Brown's supreme faith that God had ordained him for this and it would all come to pass after all. Maybe he intended from to become a martyr. Reasonable cases could be made either way. Certainly the fact that Brown decided to strike only after three new recruits arrived on October 15 is a point in favor of his hope that he had enough people for something worthwhile. And maybe it was a sign from God that he had his destiny down after all.

The initial vision, which Brown had years before all this, involved a strike to arm a guerilla army which would retreat into the mountains and essentially create a network for both assisting escaped enslaved people and freeing them in raids, sort of the Underground Railroad on steroids. Eventually this thing would disrupt enslaving so thoroughly that the system would become unsustainable and collapse.

By the time Brown told his recruits the plan, it had changed. They would capture Harper's Ferry's arsenal and gun works and seize the arms stored there...then they would hold the town until dissident whites from Maryland and what's now West Virginia could rally to his banner. They surely hated enslaving -and made noises to that effect to get their way in local politics- so this should be an easy lift once someone took the first step. Brown anticipated far greater support from the local enslaved people, who would naturally come at a full run to free themselves and their families.

That Virginia's enslaved people lived in their greatest concentrations distant from Harper's Ferry didn't seem to enter into it. When word got out, they would come and turn Brown's hold-out force into an army which could sweep through Virginia in essentially a rolling wave of liberation raids, funneling noncombatants up through the mountains to Canada. At some point this would provoke a spontaneous uprising of enslaved people across the South. The US Army might intervene, but the regulars had done nothing to impress Brown in the past so he was confident his force could evade and hold against them, perhaps from newly-constructed mountain redoubts.

In other words, Brown expected reinforcements he didn't get before going in. Then he expected more reinforcements once he did go in. Neither force materialized. Nor could Brown probably have held out much longer before losing what recruits he had. He set up shop on the rented farm on July 3 and the attack did not commence until October 16. The men he gathered to him objected strenuously to the plan, to the point of threatening mutiny. Brown reconciled them by stepping down as commander-in-chief in exchange for their promise to follow his orders so long as he proved competent. They did their best to keep busy, but Brown had a farm house full of young men who meant to see some action and who stood at constant risk of discovery by the neighbors. (At one point a neighbor let herself in, spotted some of Brown's black recruits, and suspected they planned to steal enslaved people. The group managed to persuade her otherwise.) Things got rowdy. Men snuck off to town to hang out. If he did not act soon, the option might disappear entirely or be left to a still smaller group.