I wonder if the rules of war (Geneva convention) were less about humane treatment of soldiers and decency towards civilians and more about just determining a winner and a loser.
If you're in a war and your existence is at stake, you scratch and claw and do whatever fucked-up things you have to in order to survive. But then whatever horrific things you do, the other side then does. You wind up in a never-ending war where generations of men die or are maimed and it generally screws up your society for decades. So even if you do survive the war, what was it for? You don't have anything left to do anything with.
If war is an event with rules, it shortens it and allows at least one party to go on with a functional society. And ideally, the losing side is folded into that society going forward....not ideally for them, but for the overall populations involved.
functionality > civility
OR
functionality = civility
My grandfather was an expert on the laws of war. My understanding of the answer to this question: they were/are intended to minimize suffering for both combatants and non-combatants during state-on-state warfare, and essentially take the Clausewitz-ian perspective that war is an extension of politics/diplomacy. They were not created to give more powerful sides an upper hand. The Geneva Conventions flowed from the creation of the International Red Cross, following the Battle of Solferino in the mid 1800s. That stemmed from humanitarian concerns.
They also come from the perspective of rational actors who have decided that whatever the results on a battlefield should not extend beyond the battlefield. Even among countries that "follow" the laws of war, there are individuals who break them, and the self-policing is rarely as punitive as policing by a victor against a loser. The same grandfather--who was a combatant in WWII and Vietnam (he was stationed in Germany during the active fighting of the Korean War)--noted that there were certainly atrocities/war crimes that the US committed during WWII. Likewise, I never saw him more visibly upset than discussing My Lai (incidentally, he was involved on the Military Justice side, from a high level--and angry at the Nixon administration for how it let Calley off the hook).
That said, desperate people do desperate things, and it is fairly safe to assume that combatants who view their survival (including the survival of their cause) as in the balance will be less inclined to follow the laws.
To your last point: every war ends. It is exceptionally rare (though maybe not nonexistent) that the end of war results in the eradication of the losing peoples.
