I'm under no illusions that war is not a brutal thing. No, I haven't had to put boots on ground, get shot at, and put bullets downrange personally. That doesn't mean I don't know what the results look like. I'm as familiar as I care to ever be, I know damned well it's fucking ugly, and that it's sure as shit not a game. I know damned well civilians die from time to time despite the best efforts of the US military. My point is that, as a civilization, we have had enough horrible experiences over the years to look at what happened and say "no more; never again." This is the LOAC. If you want to look at it as putting lipstick on a pig, maybe that's not far from the truth, but I'd argue it's more useful than the metaphor indicates.
I believe when you give a young man a weapon and tell him to kill that guy over there in the funny uniform, you are opening the door to Pandora's Box. You are opening the door to a Hobbesian state of nature where the strong rule the weak, and might makes right. The reason we have a LOAC is the same reason there are so many capital crimes in the UCMJ dealing with a combat situation. That high school linebacker turned PFC might be able to physically overpower and kill his 50-year-old Commanding General instead of obeying his order to die taking the proverbial hill. Why not? Because of morals. And because of the rule of law, enforced by severe punishment. Because even combat cannot be anarchy.
It doesn't mean the US hasn't done anything in the past that today would be illegal. We have. The Trail of Tears. Dresden. Tokyo. Laws evolve. Having a LOAC doesn't mean that it's going to make war into puppy-rainbow-fairy-land where no one ever dies who doesn't deserve it. Having a LOAC doesn't mean that no one is ever going to violate it. When we as an American society choose to go to war, we have bought these things, and we damned well better have a good reason for them. But having a LOAC does mean that we have collectively taken stock of this ugly beast that is armed combat, and made a value judgement that says "this far. No further, even if it costs us in blood and treasure." That is what distinguishes us from them. We have a LOAC to prevent another Dachau, Bataan, Oradour-sur-Glane or My Lai. We have a LOAC to distinguish ourselves from Jihadi John and his ilk. From Josef Mengele. From the people who raped Nanjing.
Maybe some people around here would be comfortable wearing a uniform where that wasn't the case. I wouldn't.