John Brown's raid on Harper's Ferry, VA in 1859 is significant because it helped to bring on the Civil War. In fact, the link below goes so far as to say that Brown's attempt
to liberate and arm Virginia slaves makes civil war inevitable.
The reason for this is that the raid and its aftermath inflamed the South's suspicion of the North and made it much less likely that the South would ever trust the North again.
The South saw Brown's raid as a major evil given that it was meant to make slaves rebel and kill their masters (and probably many other people as well). It is not surprising that the South would resent an effort to cause many of its citizens to be killed.
A vocal minority of Northerners saw Brown as a martyr after he was executed. This enraged the South. They felt that this attitude was common in the North. They felt the North was celebrating someone who was trying to incite others to kill them. (Think about how Americans feel about those who see Osama Bin Laden as a hero...)
Because of this, the South came to distrust the North even more and there was just about zero chance of reconciling the two regions.