Just by way of referance, the United States, in preperation for operation downfalll (the invasion of the home islands) manufactured a number of purple hearts to give to their wounded after they hit the beaches. In the years after, through Korea, Vietnam, Afghanistan and both gulf wars, they have not used all of those medals.
At the time, the military of Japan was training every man, woman and child to defend their homeland, using everything from outdated military equipment to sharp sticks. A conventional invasion of the home islands would have been a sort of unholy hybrid between the last stages of the eastern front, (Jan-May 45) and the Vietnam war.
By the time the bomb was completed, they had a total of two war shots ready. To use one on an uninhabited island as a warning shot would be unwise as it did take two to convince Japan to surrender. The allies had also, by that point, reduced every signifigant military target to rubble and had heavily damaged many civilian targets as well. Part of the reason for choosing Hiroshima and Nagasaki was that they had escaped mostly unharmed compared to the rest of the home islands.
And finally, try to see this from the allies point of view, they had been attacked without warning and without a rational cassus belli, they had seen their own tortured, starved and killed when taken prisoner, they had seen genocide to rival the holocost take place in china, tehy had seen unit 731, they had seen biological warfare used against the Chinese, they had seen the survivors from a sunked hospital ship strafed in the water, they had seen the 'competetition to behead one hundred prisoners', and they had seen the so called 'comfort houses. They had not seen any sort of resistence to the government, no massive outcry at these actions and while the majority of it may have been covered up, that sort of thing always filters home. No one in Japan seriously tried to stop this. No I ask you again, look at this from the allies point of view, no matter how horrible the thing you were about to do was, how forgiving would you be?