In asking a specific event and asking whether or not the course taken is justified or not, you are essentially asking whether or not the tactic employed was justifiable or not. Sherman's March to the Sea employed a more common military tactic known as the "scorch-earth policy" which is used in two ways:
Firstly, the scorch-earth policy is used mostly by the defending force. The deployment of this tactic is very similar, in removing any viable transportation courses (such as blowing up bridges, railroads, roads, as well as sabotaging established communication lines), as well as viable food sources, especially crops. Large mobile armies, back in those days, did heavily rely on captured enemy resources to sustain their armies, especially when transportation lines are drawn out great distances.
However, it is important to note that it was Sherman's army that employed this tactic. It gives us two important lessons. Firstly, Sherman understands that war is not always won by just battles, but that, in the greater scheme of things, it is actually a battle of spirit, population, but most importantly, resources. The destruction of the crops show Sherman's understanding that, even if the land was retaken, the crops lost would be a blow to the Southern troops, as well as seeing their native land in ashes. Secondly, Sherman understands the importance of transportation. The destruction of railroads and roads served to impede any active reinforcements in the area as well as a more concentrated attack, as the fastest ways to travel has been destroyed.
The third point of Sherman's March to the Sea is more of a ideology and policy, rather than a strategic justification. Sherman's march to the sea generally freed large amounts of previous slaves, who were left behind to take care of their master's houses while they were at war. This takes a more humanitarian approach, and casts a more positive light in the wake of destruction, in which, even though there was destruction, there was also freedom.
Sherman's March to the Sea is entirely justified, especially in a all-out war like the American Civil War. Remember, there is no Geneva Convention, no "illegal weapons", but rather a loose stratagem that asked for only one thing, to employ everything in their power to grasp the victory. History is written favorably for the victors, and justification comes along with it.
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