Given the history surrounding it's use, and I'm quite sure know it very well, why do you think it's use should be constituted a war crime?
Indiscriminate genocide is a crime against humanity. The A bombs were not the only incidents, and the US was not alone in commiting them.
There were alternatives, including laying siege to Japan via Naval blockade, but I guess the idea was to get it over with...
The A bomb is not something to thank God for.