What were the reasons against dropping the atomic bombs on Japan?

The moral
dilemma was probably the most frequently cited reason against dropping an atomic weapon on
Japan. It is fairly well countered by the idea that if the United States had not used the bomb,
they would have had to invade the Japanese homeland, which would have resulted in a tremendous
loss of life for the US military. Keeping in mind that Japan instigated the war by attacking
Pearl Harbor makes it easier to drop the moral argument.

Another reason to
choose not to use a nuclear weapon is the "Pandora's Box" argument. Once such a weapon
has been used the first time, might it become easier for other countries to follow suit? It is
true that at that time, only the United States possessed a nuclear weapon, but they knew full
well that other countries would also develop them in time. It's...