Why was Antietam a change in the strategy of the south fighting a defensive war on home territory?

Antietam was a change because the South's original plan was to play defense. Let the North smash and smash until it tired itself out and left. Antietam however, was an offensive move. This was a move to invade the North. The early victories led the South to believe that they could do more than play defense and win. Antietam was the first very bloody step to take a more offensive plan to gain freedom.