Funny you should mention "spreading democracy" though. We were told by our leaders that Iraq was a situation of destroy-them-or-die. There was no talk of democracy spread then ... it was all WMD's poised to mushroom cloud some U.S. city. If our leaders were to be believed, killing civilians en masse would have been justified. But since our leaders were either wrong or worse ... well, I'm just glad we didn't purposefully target civilians. (Lord knows plenty upon plenty have died anyway).
I'm certainly willing to concede that defeating Japan was an imperative in WWII. I might even buy that less people were killed and wounded in the nuclear attacks than would have been in any conventional attack(s) sufficient to defeat the enemy.
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