In his discussion of justice in the Enquiry Concerning the Principles of Morals, Hume tries to argue that justice is based entirely on usefulness, partly by arguing that in circumstances where justice is not useful, nobody thinks it should apply.
One of his examples concerns what he calls the laws of war. He claims these, like all rules of justice, are justified by their usefulness, and that they can and should be ignored where they are not useful. As an example of where they are not useful and should be ignored, he presents the case of fighting enemies, barbarians, who do not themselves follow the rules.
Obviously, this seems relevant to current circumstances; many people seem to agree with Hume's reasoning, and favor applying it to our dealings with terrorists. Thus, I thought this passage would be a good one to spend some time on with my students. I was a little disturbed at the result.
One student noted that the laws of war are presumably justified on the basis that following them reduces the damage done by warfare, and that one side following the rules would still reduce the damage somewhat, if not as much as both sides following them. Thus, she said it seemed that only one side following the rules was still useful.
However, she was a lone voice in that cause. Pretty much everyone else who expressed an opinion thought it made no sense to follow the rules if your enemy didn't. Indeed, some seemed to think it made no sense to follow any rules of war at all; that if you're at war, you should do whatever will most quickly and efficiently bring victory.
These are university students, and so to be expected to be liberal-leaning, and they're in deep blue Massachusetts. I did not expect to find such sentiments. I don't know what to think about this.