What America lacks is any knowledge of what is and isn't a war crime.
What America has is a chorus of howler monkey politicians who keep dehumanizing their enemies.

Put those 2 together and you get a populace who thinks it's OK for US to do any bad things we can imagine, because we do bad things in the name of such epic goodness.

It gets much closer to home than justifying war crimes. Look at how we treat our own "free" citizens. Look at how our prisons are over-full, and we're contracting NGO's to run prisons for us. Look at how we incarcerate more of our "free" citizens per capita than any other country we have data on.

The vilification of other humans in America is a problem that's never really gone away. From "all men are created equal" originally meaning only white men who owned land to today, we have this problem.