We know that the Nazis were nowhere near building an atomic bomb themselves and we know that the US government was categorically aware that there was no threat of an atomic bomb from Germany by Spring of 1943. They started building the bomb because - apparently - they were in a race with the Nazis. They weren't, yet they carried on building one and never bothered to tell anyone it was a one horse race.
The myths about the US Atomic bomb and WW2 are just that. Anyone who's read the history knows the truth, it doesn't take a great deal of common sense.
There may be debate, but most scholars outside the US have come to the conclusion that it was far from a necessary measure and of course the consequences have been dire for all of us...