Let's suppose one of John Brown's intended victims (for argument's sake, James Doyle, the first) somehow gets advance word of the hit squad Brown is putting together. In response, he calls in some favors, fortifies his cabin, and lies in wait. In short, John Brown and his men find twenty armed and motivated people waiting for them. After a brief struggle, Brown and his followers are all killed or captured.
So what does this mean for Bleeding Kansas and the leadup to the Civil War? Where are we without Harper's Ferry?