Answer:
The ethical problem of saving American lifes by destroying Japanese.
Step-by-step explanation:
Historians agree that the main reason of the US to use atomic bombs was the desire to save as much American soldiers as possible. We shouldn´t forget that untll then the Japanese government and its army (not their civil population) showed an iron (samourai) will to fight till the very end.
After the bombing of Hiroshima there was no sign from the Japanese government to surrender. Therefore, the US decided to continue and bomb Nagasaki three days later.
As Eric Hobsbawm pointed out in The Age of Extremes, world war II was the first democratic war in the sence that civilians all over the world became victims on a large scale too.