Wars between states have been around for at least five thousand years. These laws are brand spanking new. Genocide was considered completely fine just a few centuries ago. So was collective punishment.

Many of these treaties get violated in the heat of war, and that's understandable. E.g. bombing cities was banned before WWII. Chemical weapons were banned before WWI. When both sides of a conflict violate the rule, it's unlikely there will be any consequences.

All democratic societies can do is systematically take the side of the most ethically behaving party in any conflict without any preference to historical ties and race. And occasionally sue their own government when it violates these rules.

Very lofty, doesn't happen much. Though arguably Ukraine is a good example here. EU countries had much better ties with Russia, that's where the gas came from. But they support Ukraine, even after they blew up the pipeline. That's very encouraging.