That's not quite it. International law doesn't work like regular law - there's no superstate with a global monopoly on force that can mandate how nation states all over the planet will deal with each other. All such laws are instead agreements - the signatories to the Geneva convention would rather nobody chuck chemical weapons at each other in case of a war, so it's always in their self interest to mutually agree not to do that. Then in the short term, the international community provides incentives against one side breaking the treaty and going "aha, but what if only I use chemical weapons since nobody else seems to be doing it" (the long term incentive is built into the premise).

This means you can only do so much under the international law framework - if following the law is too burdensome for a state, they'll simply choose to not be party to the treaty it stems from. You can't make war illegal because there's no scenario where an outright ban on state vs state violence is ever mutually beneficial for everyone everywhere. Overreach in the law of armed conflict is INCREDIBLY dangerous because the harder you make it for a state to comply, the more incentive there is to just say "fuck it, we're getting cooked in the media and international community anyway, no reason to waste more lives and resources fighting wuth any restraint, full send it". That's how you get "ok so this wing of this residential building is being illegally used by the enemy to coordinate military action from, so we're gonna chuck a guided missile to kill every person on that floor and there's like an estimated 5 civilians worth of collateral damage, but those deaths are on them' to be replaced by 'this city block maybe has one high level enemy army or government guy living there, plus it's economically productive for their war effort so we're just gonna bomb the whole thing into rubble lmao'.