One example of what could happen if you break the war rules is we make the earth uninhabitable.
Mankind has developed such powerful weaponry like atomic bombs governments have to place restraints on themselves to ensure they don’t kill us off.
The honor system seems like a weak solution imo but it is what it is.
Some of it is proposed by world powers in the name of humanity. Part of the motivation is to also maintain their dominance. Like with chemical weapons. A smaller government may not have large ground forces but can do serious damage with sarin gas. Putting a restraint on those smaller countries will keep them from becoming a contender on the world stage. It will also prevent the destruction caused by sarin gas.
But I don’t buy into it being more humane than having a frag grenade give you permanent brain damage for the rest of your life.
If you and me decide to fight, we might agree on some rules beforehand. No biting, no eye poking, no kicks to the nuts. Once the fight is going strong, somebody might be tempted to break those rules. Since (in this case) there’s no “world policeman” that can step in, the only punishment for you breaking the rules is that it might make me decide to break the rules too. Or maybe other people might jump in if they think the move you pulled was dirty enough.
Same thing with war.
There are two kind of things that are usually “banned” in wars:
1. Breaking of conventions
2. Disproportionally cruel acts
The consequences of the first one is simple: If you fake surrenders too often, peoples won’t believe you if you ever need to actually surrender one day and will kill you on the spot. If you use peace negotiations to ambush an enemy commander, good luck with future peace negotiations. Obviously, no one is naive, peoples will try to cheat if they think it gives them a significant edge, so you need to be prudent enough so that it’s never too tempting for your enemy to backstab you.
Additional consequence to the first one: breaking rules makes you less legitimate to call other on their rule breaking. Wars being a complex network of alliances and promises, and winning the wars by backstabbing will undermine your legitimacy in the post-war period.
For the second one, one big reason is soldier’s moral, which is central in having an effective army. It takes a lot to convince a sane human being to kill another, and it’s much easier to do so if you convince them that they are “the good guys”. And while “fighting evil with evil” or “revenge” justifies a lot of thing, every soldier will have a limit past which they might start thinking “are we the baddies?”. Following a set of rules is a very practical way of convincing peoples that “it is morally acceptable to follow order”. If the acts you are doing are not banned by the convention that banned the war crimes, then that means it’s morally ok to do them at war, right? And to maintain this, you need to actually not do those disproportionally cruel acts, or at least to not get caught when you make them.
Additionally, the second one helps to create some feeling of safety for your soldier that they most likely won’t die it the most horrible ways, and and helps at building propaganda so that the civil population continue to support the war.
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