“Now we are back to the atomic bomb! A final blow to make Japan surrender, or a warning to Russia?”
It was probably both: at the time America was losing thousands of marines every week in the war against Japan, and it was believed that the Japanese would fight to the last man to defend their homeland, since surrender was the greatest dishonour imaginable in their culture of Bushido. The Bomb was used to end the war quickly by offering a reason for Japan to surrender without dishonour. Atomic weapons cannot be defeated by conventionally armed forces, however brave, so this offered them the opportunity to surrender without loss of face. The use of the two atomic bombs saved at least 5 million lives, most of them Japanese. Had I been the American President at the time I would have made the same decision, and slept well for the rest of my life.
It is worth remembering that for several years America was the only nuclear power, and did not use their advantage against their cold war enemies, while they had no nuclear arms. If Hitler or Stalin had achieved the same advantage, would they have been so restrained?