Dont use the wording "war crimes". No one during the civil war was guilty of
The conventions (saint Petersburg, Geneva... ) are rules about warfare signed by sovereign states and they are only in effect during wars between the states.
So if a danish soldier in Afghanistan kill a captured Taliban fighter, that would not be a war crime. Since the Taliban have not signed the relevant conventions. But it would still a crime by danish law.
And that is naturally ignoring the fact that they did not exist back then.
Show me the treaty that define war crimes and signed by both sides. If not stop using that wording... it is anachronistic.
The general order you quote do make plundering a crime by military law... but that do not make it a war crime.