I have just translated two chapters on the Laws of War written (in French)by a German expert on diplomatic law in 1801. I have sent them in to Max Sewell and they should be appearing on the other side shortly.
The gist of it is that there are natural laws which apply and also custom and specific treaties. For every rule there are so many exceptions, notably military necessity, exceptional circumstances and reprisals, that the whole concept becomes rather meaningless. The author admits that one way of identifying the laws of war is that one side always accuses the other of breaking them.
It is clear that there was a concept of a clean sporting fight with everyone shaking hands at the end. It is also clear that it was never an actuality.