Sherman's March to The Sea is a decision for 
USA, when the Civil war is raging against 
Confederate States of America. If it is not used during the Civil War it can potentially be used in a second war against CSA.
It requires that USA occupy the 3 provinces Atlanta, Augusta and Savannah. It will decrease the population in Atlanta by 30%, and the population in the to other provinces by 10%. On the other hand, it will give CSA 4 War exhaustion.
The war exhaustion will help USA win the Civil war, but will damage USA in the long run, if they win the war. Unless USA is about to lose the war, it is advised not to take the decision because of the long run damage it will do.