Humanitarian law, for the most part, serves to help people feel better about horrible tragedies. It does not serve a meaningful deterrent effect. I recall watching a lawyer discuss all the work he had done in investigation and prosecution after the Rwandan genocide. I asked the question, "Do you think this prosecution did anything to deter such actions in the future?" He seemed somewhat surprised, and said, basically, "No, not really."

In other words, humanitarian law has the major caveat that it's only enforced against the losers, or relatively powerless winners. So looking at it from a game-theoretic perspective is not terribly productive.