How could the Union invade it's own country? Which the South WAS, because the people there had REBELLED against the Union. Perhaps you are forgetting Fort Sumter, in which South Carolinians fired upon Union Troops? http://www.civilwarhome.com/ftsumterevents.htm
If you feel that his march was barbaric, wasteful of civilian lives and property, and that it's purpose of preventing casualties by shocking the south into submission was not a valid one, I have two words for you:
Of course, anything I say will not change your opinion, so I will leave it at this: Read a biography or two of Sherman, I don't care which, with an eye towards the actual man behind the story. Try to see who and what he was, before the war, and try to understand what could have led him to the policy he adopted. I don't have anything further to say on this topic.