I was addressing only the strategic destruction of civilian society, as carried on by Sherman. This is a war crime, no matter what the political or social forces which led to the formation of the CSA.
All war is a crime. But either war is obsolete or men are.
You make a valid point. Just war theory involves two logically separate components, jus in bello and jus ad bellum. The first governs conduct of soldiers during wartime, and the second governs acceptable reasons for engaging in warfare. They are logically independent in the sense that a soldier fighting in a war that was illegitimate in its conception still can fight in a manner that is honorable and legitimate. That soldier is still entitled to the protections of the laws of war (such as those codified in the Geneva Conventions, for instance).
Conversely, a soldier fighting for a country that was legitimately justified in going to war can commit war crimes. The question of whether Sherman committed war crimes, then, is completely independent of any justification for fighting the war. The laws of war are indeed fairly clear - it is impermissible to intentionally target civilians. Sherman’s army clearly did target civilians intentionally. The only question for Sherman’s personal responsibility is whether he specifically ordered such targeting. It certainly seems to me that he did not specifically order civilians to be targeted, but that he must nonetheless be held responsible for the actions of his men. He failed to do enough to maintain discipline to prevent widespread targeting of civilians and he should have realized that his plan for the “March to the Sea” had great probability of resulting in civilian casualties.