They wouldn't have aimed for America.
If Japan had a nuclear bomb, they would either have dropped the bomb on Hawaii or on the Chinese mainland or possibly headed further South in the Asia Pacific region.
People seem to forget the atrocities committed by the Japanese during the war. They would not have stopped. This was a country that saw fit to have soldiers fly suicide missions. Loss of lives meant nothing to them. They didn't care about the loss of lives when those two bombs were dropped. They were only concerned about how they would impact their war machine. They committed hundreds of atrocities.
And these atrocities were not just against soldiers, but mostly against civilians, the Nanking Massacre, which killed around 300,000 Chinese citizens springs to mind, and the Three Alls Policy that saw nearly 3 million Chinese civilians massacred by the Japanese in a scorched earth policy lasting several years.. Those are just two out of the
many many massacres committed by the Japanese forces, which resulted in the deaths of millions.. The torture, rapes, forced labour, mass murders, use of biological weapons, and other atrocities (even strapping Chinese citizens to crosses and using them as bayonet practice and burying people alive, to name a few, and medical experiments on civilians).. They are still to apologise for all the crimes they have committed. Comfort women are still to receive an apology for the atrocities they were made to endure. Japanese history lessons still do not address the role of Japanese forces in the horrors they perpetrated on innocent civilians during the wars they started against numerous countries in the Asia Pacific region.
They would not have stopped, regardless of requests or demands to stop. Telling them the US had an atomic weapon would not have stopped them. It would have spurred them to kill even more people as quickly as possible. Even after the bombs were dropped, it nearly did not stop them. Many suspect it was
Marcus McDilda's false confession, obtained under torture, that the US had more bombs and that Tokyo and other cities in Japan were next, and the impending threat of a Russian invasion (the Soviet Union had declared war on Japan), that may have stopped them in the end. Otherwise they would not have stopped and they would have massacred millions more.
No one agrees with the use of nuclear bombs against civilians. But it had literally gotten to the point where the US felt it had no other choice. And frankly, they probably did not. Should they have aimed for civilian targets? No. And these bombs were not aimed at citizen targets. The Hiroshima and Nagasaki had the least in regards to citizen costs, hence why they were selected. But it took a long time for Japan to finally surrender. It was only when they were left with no choice and they realised there was no way out, did they surrender. The alternative to the bomb would have been a Soviet and American invasion, from both sides, which would have resulted in a much higher cost of civilian life.
I don't agree with the use of nuclear weapons. But I also recognise why they felt it was necessary at that time.