As I have said before, I think that the moral rightness of the bomb drop on Hiroshima is contingent upon two issues:
(1) Could we have achieved the surrender of the Japanese without either the bombing or an invasion if we had chosen to negotiate a surrender rather than insisting on an unconditional surrender?
(2) Is there any reason why a conditional surrender that would be acceptable to Japan should not be acceptable to us?
If the bombs were truly the only way to prevent a land invasion, or if a conditional surrender would have had horrible consequences, then the Hiroshima bombing was justified. If the Allies had good reason to assume either of those two things, the bombing was also justified - or, more precisely, the decision to bomb was justified given the knowledge that they had at the time. Otherwise, I think that the bombing was not justified.
There are basically two points that I think are important to consider whenever thinking about Hiroshima )or World War II in general):
(1) Is the demand for unconditional surrender a good thing to have in a war? Perhgaps in some wars it is the right way to fight, but I think that the tendency amongst some philosophers and pundits to look at unconditional surrender as a necessiity in any modern war is rather misguided/
(2) Hiroshima and Nagasaki were extreme decisions under extreme circumstances. They ought not to be considered universal precedents, as some pundits and commentariat would have them be (e.g. the desire to flatten the "Sunni triangle" in Iraq).
That is all.