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You've admitted that wiping out a city because someone tried to kill you is a disproportionate response. What you don't seem to understand is that wiping out civilians en masse is always a disproportionate response.
I completely disagree. It would have been completely proportionate, although no less immoral, for the Japanese to have fire bombed all US cities (if they could have) after how the US treated mainland Japan in WWII.
A proportional response is basically along the lines of an eye for an eye. What it precludes is a life for an eye.