Are war crimes not war crimes if they’re sufficiently "necessary?"

In the comments to the Hiroshima thread below, djw asks an excellent question, in response to justifications for dropping the bomb on Japan because doing so arguably saved many more lives than were killed by the atom bombs:

King Rat, do you think our definition of war crimes (as codified in the Geneva conventions) should be amended to remove intentionally mass-murdering civilians from a list of war crimes when the utilitarian calculus tilts against it?(I’m not trying to be snarky, I mean this as a serious question. Something about Truman as war criminal seems not quite right to me too, but the alternative seems much worse).

Justifications of the terror bombing of cities come down to the claim that it’s OK to intentionally kill large numbers of civilians if doing so produces good results on the whole. As djw says, this would seem to require the defenders of dropping the bomb (and of course the defenders of the firebombing of Tokyo and Dresden and Hamburg) to explain if they favor a general utilitarian exemption to the concept of a war crime.

Another way of putting this is to ask if there’s a moral difference between marching into a city and systematically rounding up and machine gunning its populace, and getting exactly the same result by dropping bombs from planes. For reasons that are hard to understand rationally but which seem to have great psychological force, violence at a distance seems different to many (most?) people than violence at close range, even when the intention and result is exactly the same (lots of dead people). Or to put it another way, as William Ian Miller points out in his book Humiliation, violence at a distance somehow seems less violent and therefore less morally problematic (relatively speaking of course).

I’m guessing no one would defend the idea of rounding up and then machine-gunning the entire populace of a large city, no matter how beneficial the consequences of this act were purported to be. But the terror bombing of cities during WWII continues to have many supporters. Like djw I’m trying not to pose this question in a rhetorical way — I’m sincerely curious how people make these distinctions.