Straw man argument.   Not even a good one.    

Even if you can justify Hiroshima on the basis of wanting to end the war, Nagasaki (3 days later) cannot be explained by the same reasoning.    Nagasaki (using a completely different nuclear bomb) was only a smaller civilian casualty count because of luck, not intent.     It's hard to justify detonating a second bomb days after the first as if Japan was a threat for retaliation (they weren't).

This issue will always be debatable by history.    Of course, the winner writes history.    Put the same actions in the hands of Eg:  Nazi Germany, with Chicago, or Philadelphia as the targets and I'm sure that the feelings and "history" would be much different.