At what point did the American Civil War became a question of "when" rather than "if"?
John Brown’s Raid on Harper Ferry could be considered the turning point in the American Civil War when arm conflict seemed all but inevitable for several reasons.
First, it represented the nightmares of the southern people come to life; a slave rebellion led by an abolitionist. There was already a large amount of distrust between the north and south, but after this attack the south saw themselves as under siege by their neighbors to the north. The way that Brown was idolized in the north also seemed to confirm the fears of southerners that the North was behind the attack at least morally.
In response, the south began revamping its militia system to combat future threats. Weapon stores were resupplied, men were trained, and logistical plans began taking shape. This was the beginning of what would soon become the Confederate army.