The US were the only one to use atomic weapons on a already defeated enemy/at all.
Source: My grandfather had orders to go and do exactly that when the dropping of the bombs ended the war.
The US had already been ridiculously effective using firebombing to level Japanese cities with their B-29s - so much so, they actually had to consider slowing down/changing targets to leave enough behind to use the Atomic Bomb on: there was almost nothing left worth hitting in strategic terms. By the time the bomb was dropped Japan was largely a beaten nation already considering surrender, Tokyo a smoldering rubble pile save for the Imperial Palace.
"The bomb simply had to be used -- so much money had been expended on it. Had it failed, how would we have explained the huge expenditure? Think of the public outcry there would have been... The relief to everyone concerned when the bomb was finished and dropped was enormous." - AJP Taylor.
Of course no one can say with certainty, but I certainly don't consider the answer to this question to be a simple one.
Even after Nagasaki, it took personal intervention from the Emperor and the foiling of an attempted coup for Japan to surrender.
Of course, dropping the bomb and developing the bomb are two distinct, albeit related, ethical questions.
To quote Wikipedia: "On the night of 9–10 March 1945, Operation Meetinghouse was conducted and is regarded as the single most destructive bombing raid in human history. 16 square miles (41 km2) of central Tokyo were annihilated, over 1 million were made homeless with an estimated 100,000 civilian deaths."
The bombing campaign leading up to the atom bombs specifically left about 5-6 cities relatively untouched. There were still major strategic targets left in August. They did this to test their effectiveness on cities, as a demonstration to the soviets, and to destroy morale.
Demonstrating their effectiveness to the soviets is why they didn't drop them in Tokyo bay.