The Manifest Destiny doers didn't know it, but most of the Americas had been cleared a hundred years earlier by disease. Some estimate 95% fatality rates.
So, is OK to decimate everybody left because disease had already killed a lot of the native people. So if another country had come to conquer the European nations after the black plague and committed genocide would it have been OK? Otherwise I'm not exactly sure what is your point.
Is like you are trying to say, hey, I only killed 4%, the other 95% were already dead so is not so bad.
There's no one saying it is okay here. Everyone agrees it meets the modern definition of a genocide. But I'd would certainly need some evidence that the US was responsible for 80% of the deaths of the remaining native Americans. I looked around for these numbers and have never seen anyone claim this.
