To be fair, this isn't entirely fundie, as you're only likely to be pulled up for war crimes if you lose (viz. as Solomongrundy says above, Bush and Obama and the whole "enemy combatants" thing and arguably drone strikes on civilians.)
If you win, or you are big enough and powerful enough, you can get away with anything. The fire-bombing of Dresden in WWII was arguably a war crime. US soldiers and officers committed war crimes in Vietnam also. But the folks responsible will never see a war crimes tribunal because nobody's powerful enough to enforce the writ of the Geneva Convention here.
Secondly, the sorts of folks who the West is likely to go to war with aren't likely to respect the Geneva Convention anyhow, therefore there's little risk of them doing the same to Western personnel if the West maltreats prisoners from ISIS etc.
TL;DR - Fundie in saying that war crimes are not a thing, but not fundie in saying that, depressingly, if you win the war in question or are large enough that nobody can enforce the Geneva Convention against you, you'll get away with it every time; Trump might commit war crimes but will never see the inside of a courtroom.