Today, we choose not to use such techniques. You apply today's standards to the 1860's. You need to judge the 1860's by the standards of those days. The Articles of War have some interesting admonitions in them, well worth reading. But it must be remembered that war is very much an exercise in natural right. To establish a "set of rules" governing the belligerents is a wholly artificial construction. When you get right down to the basics, war is two guys duking it out until one concedes or is killed. There are no consequences for the victor. There is no justice for the loser. It is the law of the jungle.
Will your beloved Lieber Code work then? As surely you know of its Article 16:
"Military necessity does not admit of cruelty - that is, the infliction of suffering for the sake of suffering or for revenge, nor of maiming or wounding except in fight, nor of torture to extort confessions. It does not admit of the use of poison in any way, nor of the wanton devastation of a district. It admits of deception, but disclaims acts of perfidy; and, in general, military necessity does not include any act of hostility which makes the return to peace unnecessarily difficult."