Because they were fighting a different kind of war that they were not use to. The men were trained to fight for battles like in Europe in ww2. They didn’t not teach the men how to fight a counter insurgency war. They would be trained in taking and holding objectives, then be deployed just to search a village not really knowing exactly what to look for, then round up the villagers and burn their homes and food supply.

It probably came from two main things. The frustration of not getting anywhere, they would patrol the same area for weeks on end, and it would destroy morale to lose a man taking a hill, and then a week later returning to the same spot just to do the same job you already lost guys doing. The men would grow a real resentment to the VC/NVA, trouble is how do you tell a VC from a old farm hand? Especially when two villages back you found a similar man that was hiding 20 AKs under his bed.

And they were kind of told to. Not really, but. America was not ready-or expecting a 10 year for a war run by politics, so their reaction when they realised that they needed a new strategy was body count. How do you get rid of an invading ideology? Kill so many of them that they don’t return. The brass really hung onto this. The soldiers were taught that if you kill a civilian it gets counted as VC, and goes onto the body count. I think they became to numb between their own anger and frustrations of their own war, and the orders from above. They did not care if the war ended or not, just let them survive their 12/13 months and get home, whatever that would take.

But there were war crimes from the US in ww2 as well, Vietnam was just the first uncensored war on TV. There were newsreels of US in Germany shooting surrendering unarmed Germans, it was not shown on TV. The US has always been looked at as the good guy in every war, but I don’t think that there is one at all. I love America though, and it is war after all.