Just War, first articulated by St Thomas Aquinas, made clear you go to war when your cause is ‘just’ and you conduct war in such a manner as to minimize suffering of the innocent.
“•The war must be fought proportionally.
This means do not use more force than necessary or kill more civilians than necessary.”
http://h2g2.com/dna/h2g2/alabaster/A644672
http://www.h2g2.com/approved_entry/A644672
http://www.freerepublic.com/focus/f-religion/1996394/posts
It seems to me that the Allied bombing of cities in WWII, and the bombing of Hiroshima and Nagasaki, were not justifiable under the Just War rubric. I don't understand why the nuclear weapons weren't used against military targets.
The moral question would be whether the killing of the civilians was directly intended or was it foreseen but not intended. The people who justify the bombing of Hiroshima would have to show that the killing of the civilian population was not meant as a means to an end.
In other words, what was the target?
If the target was the city of Hiroshima, together with its population, the act is condemned.
On the other hand, some have argued that the target was certain massive military assets in Hiroshima. In this case, the destruction of those assets would have been justifiable. They would say the collateral deaths were first of all not intended, and secondly, proportional to the aim of bringing the much larger mass killing of WWII to a quicker end.