The current Rules of War were derived from the experience of Europe from about the year 1500 to 2000. In that environment you had a) ordinary people who lived there & didn’t much give a shit about the boss du jour , b) bosses who wanted to rule ze vorld, c) fighting folks glad to work for a boss if it meant a little more cash & maybe a chance to hurt other people. Also, suicide was never part of the social / ethical / religious tradition. As a general rule, people always tried to avoid dying. Finally, up until very late, they lacked the capability to kill ALL of a population or render ALL the land useless.
So in that environment, after hostilities ended most of the ordinary people & their farms & towns & such would still be there & they’d go back to their lives. The losing boss would be replaced with another one, and the majority of the fighters went back to not being hostile with anyone, while a few turned to crime to keep getting their violence fix. Then, 20-50 years later, let’s do it all again.
In that environment, the goal is to contain conflict to “it’s just business” as much as possible. Killing all the serfs, or torturing all the enemy POWs is shortsighted in that it doesn’t really improve your ability to win, but it does make managing the post-war environment a lot harder.
Now fast forward to the 21st century. Those sorts of conflicts are largely passe, mostly becasue the serfs now rule in most modern countries & they’ve broken the code that wars are fun for bosses but not for serfs.
So conflicts now are either between countries that haven’t gotten that clue yet, or people fighting what are mostly wars of revenge. Most Palistinians couldn’t give a rat’s ass about Judaism versus Islam. But the care a lot that some guy from Them shot their brother & bulldozed their house. Likewise the typical Israeli is a lot more bent about dead neighbors than they are about the details in ancient books.
In that environment, the Rules of War are more, not less important. Only when the cycle of revenge is broken do we begin to have a fire small enough that we can put it out and/or they can let it go out.
Finally, we see assymetric warfare with non-state actors, e.g. Al Qaeda. The traditional Rules of War recognized that both sides faces the common problem of being on the receiving end of the other’s violenec, and of having to manage a post-war environment after they won (they hope). That drove a certain amount of rational calculation into both side’s thinking.
From a Western POV, Al Qaeda is essentially nihilist. All they want is dead Westerners. Their concept of an ideal postwar environment IS one where everyone is either honestly converted, or dead. And they expect that to be a 2%/98% proposition. They also hope/expect God will help them with the pesky logistics of killing hundreds of millions of people. (Can you imagine the stench)
So they have little incentive to play be the traditional rules. And which they see as stacked against their style of warfare.
A valid question in any contest is whether to play by the Rules when your opponent does not. Setting aside the legal issue of treaty obligations, it comes down to a complex balance of benefits. But for any ratioanl state actor, the practical reality that you’re gonna get stuck managing the postwar environment, which will last a lot longer than the war ever did.
So assuming your country’s very survival doesn’t hinge on this decision, you take the long view & play by the Rules. When your survival is at stake, then you cheat. Hence the Cold War doctrine of Massive Retaliation.
And there you have it: a potted one semester course in the Rules of War.