Well, when it comes to firearms, incapacitation doesn't work quite like the movies show. You can't just hit someone in the arm/leg every time (aiming is harder than that), and even if you do it will fairly likely kill them anyway. If you really want to priorize incapacitating enemy soldiers, you're going to have to use different weapons than firearms, weapons that are probably shorter range and slower. That isn't a practical option when the enemy soldiers have guns themselves.
I'm all for peace but telling your own soldiers not to kill could be a very good way to get many of your own soldiers killed.
We could use some serious development in non-lethal weapons, but I'm afraid it might not solve anything - non-lethal weaponry is by definition more complicated by lethal, and thus any technology designed to stun could be repurposed to kill with more effectiveness than the bloodless variant. We'd probably need to reach the level when there would be no practical difference of effectiveness between the two types.
One of the fascinating things to come out of the widespread distribution of non-lethal weapons to police has been two effects:
1 - police are more likely to use non-lethal weapons, even in cases where no weapon at all would work fine, because the long-term consequences of using a non-lethal are minimal
2 - in cases where people do die due to being attacked by a non-lethal weapon, there's generally quite a bit of moral outrage that the weapon was used at all, even if it was well warranted
I suspect if soldiers were issued stun weapons tomorrow, we'd see both of these come true and people would still cry foul when some percentage of targets inevitably died as the result of the use of the non-lethal weapon, and rules of engagement would be vastly changed to virtually eliminate the idea of escalation of force, just go in shooting and sort it out later.
There's a difference between an army and a police force. Police are there to enforce the law and neutralize criminals.
Generally speaking, the police don't want to kill anyone. They want to capture the suspect and move on. Back in the day, non-lethal force meant beating the guy with a big stick, but liability became an issue. So now we pull guns on dumb kids, and a jumpy cop results in a dead dum kid instead of a broken arm.
Armies are different. Since the US Civil War, total annihilation of the enemy's ability to make war has been the rule of the day. You err on the side of killing, becuase a stunned enemy lives to fight another day.
So, on your last point, you're sort of missing an important part of absolute war:
Against a fair enemy, the "ability to make war" becomes the problem of destroying capital of the enemy (factories, railroads, etc.). Against an enemy that doesn't have any sort of indigenous weapons production or roads to speak of, the "ability to make war" consists of the problem of destroying the manpower of the enemy.
So, we notice that while we bombed Germany and Japan, less than three decades later they were banging along--and we notice that when we spend a decade in Iraq and Afghanistan, we end up just playing whack-a-mole with hapless militia.
Well yeah, but there's a powerful argument that militaries have been increasingly acting as over-amped global police forces. Part of this is a growing queasiness with war, like the OP, who don't understand what it is. It's why we use precision munitions and snipers instead of just firebombing cities. It's why we go door to door to "detain" people instead of just shelling the house.
The next logical step, and the one the military is very actively pursuing, is nonlethals.
That would be uber cool. Unfortunately, we don't have any that rival the effectiveness of lethal ones, thus if one were to use stun weaponry, one would lose to an enemy that decided not to.
To add on to that the complexity of developing such weapons grows considerably when you take into account the uniqueness of each target - various resistances, adrenaline, different body composition, hit area and so on. If you fail to incapacitate the target they might retaliate lethally.
Which is actually possible with lethal weapons too - people don't always die from a shot or two and not always instantly. So to ensure the threat is defused the weapons must be very effective and the best way to ensure that is by making them as lethal as possible.
Indeed. And on the other side of the equation, if you overdo your non-lethal weapons, they can easily become lethal. Tasers have already claimed many lives. That ball of fast-setting cementy thing you shoot at your target may solidify in a way that chokes your enemy to death. And this is all discounting the possibility of a disabled enemy falling to his death by accident.
Effective non-lethal weapon is a very narrow target to aim for.
I'm all for peace but telling your own soldiers not to kill could be a very good way to get many of your own soldiers killed.