During WWII most warring nations used such weapons to an extent. Regretfully, only those who lost were punished additionally for that. Those who won got a free pass, and that is why we may yet see some WMD usage, especially now that MAD is gone for good.
It has always been the case that the loser is punished and the winner isn't and always will be. That's why when you're fighting a war you need to do everything you can to make sure that you win.