The reason that the US is marked for eternity as another brutal monster like Rome, is that they very well knew the effects of the bomb before it was used in Japan, since they already had tested it. And, the scientists who created it knew what the yield would be!

Nobody and nothing except G_d could forgive this act!

It was and reamains a deed of such enormous evil that it's perpetrators today labor to disarm any other nation trying to acquire such weapons.