Bryan Caplan runs through eight reasons we see the My Lai massacre as a war crime but believe nuking Hiroshima was justified. His conclusion:
The soldiers in My Lai murdered people they could see face-to-face. The crew of the Enola Gay dropped a bomb from a high distance and flew away. Needless to say, if [this is] the true explanation…either Hiroshima was a war crime, or My Lai wasn't.
Jason Kuznicki broadens the argument:
A parallel reasoning also explains why Michael Vick became a national pariah, while the meat industry went unaffected. Whatever you think about animal rights — and I’m personally a skeptic — the inconsistency remains. It’s likely due to our varied intuitions about near and far acts of violence. And, because I can’t resist some shameless self-promotion, this month’s Cato Unbound is highly relevant as well.
The Dish 