Of course they do. Europeans had rules of war for centuries before WWI, when everyone turned into a nigger.
There are specific rules on how a war needs to be fought, some of them coming from tradition, some of them coming from international agreements. You should study the history of warfare.
OP is just wrong. The Hague convention established what constituted a war crime long before WWII.
The moment you define a "war crime", you define war as not a crime. If you're going to commit a crime, you go all the way and you make sure you get away with it. There's no such thing as a war crime.
There are specific rules on how a war needs to be fought, some of them coming from tradition, some of them coming from international agreements. You should study the history of warfare.
OP is just wrong. The Hague convention established what constituted a war crime long before WWII.
War is not, and has never been, a "crime". Why would war be a crime?
You should read more history books and consume less mainstream media.