If that was the case the My Lai Massacre would be meaningless, because the entire war would have been that.

The entire war was that. https://en.wikipedia.org/wiki/Agent_Orange

Genocide is usually not an efficient way to wage war. Even ancient armies usually did not slaughter entire towns, not because of ethics, but because fighting sucks and if you slaughter everybody you come across, nobody is going to surrender and every person you come across will fight you to the death.

I say usually, because there are some exceptions and nukes are one of them, they make genocide a viable war strategy. But even nuking Vietnam would not have worked, cause you will simply get nuked back.