You bring up some great questions George. Can there be a war crime without an established code? I'm not a legal scholar, but overall, I would say -- yes and no. (LOL, confused yet?) Let me explain:

I say "yes" in the sense that human societies have functioned for thousands of years (though less so now) without written laws to dictate every action. Rules or "laws" could be followed and enforced by communities without a strict legal code as a guide. A person or group of people could be "guilty" of a "crime" that really only was the breaking of a traditional standard. In short, a criminal action is not only made criminal by the literal fact of it being recorded on stone or paper of its criminality. An act is criminal if a majority of people in a community agree that it violates their common and accepted code. Imagine the social code of not cutting in line---there may not be a written law, but a violator can be seen by all others as a cheat.

I say "no" because without a written declaration of what constitutes a crime, a group of people may challenge the assumed standard, thus fudging or ignoring the act as a crime. A written legal code presents a set of rules beyond question (at least one would hope) for everyone. A written legal code establishes a standard that is less easily violated, ignored or challenged. To recall the social code of not cutting in line . . . if someone cuts into line and a person challenges the act, the violator may literally ask what "law" they had broken. If the violater chooses not to recognize the social standard, and if there is not a community enforcer tasked or interested in regulating that standard, where is the "crime"? The idea that "cutting is not polite" may be true, but it does not really make it a punishable "crime."

All of this comes together for Sherman and the Civil War by asking whether Sherman or his men violated any actual written laws. Some surely did, as there were cases of murder that were reported and, I believe, even punished. Whether Sherman's actions were shocking, repulsive or even "dishonorable" is important in understanding social standards of his day, but does not necessarily establish guilt of a punishable "crime."

Since Sherman was on the winning side (which also happened to be proportionally larger than the losing side), his actions were accepted, even if they perhaps violated previous standards. (The same has been said about America's use of the atom bomb.) This is not an excuse, although it is something that has happened, and will continue to happen, through history as people, communities, and nations respond to new threats and problems. On the other side of the coin, had the Confederacy won, and men like Bloody Bill Anderson and William Quantrill survived, would they have been punished for war crimes by the Confederate government for acts they committed during the war against Northerners?

As for Henry Wirz, I admit I am not too knowledgeable of his exact case. I am familiar with his story, but I have not investigated the exact legal arguments made by the prosecution. I believe that the basic complaint leading to his prosecution stemmed from the idea that he purposefully neglected Union prisoners in a cruel and unusual manner. Again, I don't know the exact legal argument, but I assume it followed some line of humane treatment of prisoners. This touches back on one of my original points about Sherman --- we should try to separate which actions were meant to and/or did benefit a military cause and which actions were only spiteful and cruel in their own sake, with no military value? I believe that is the important consideration and something that doomed Wirz.