(in english only)
The debate is still going on, regarding the final act of World War Two: was the atomic bomb use "justified?" As a "global intelligence" website (stratfor.com) shows in a recent article* the debate is really about the moral character of the United States, more than the use of the Bomb itself. How the USA can be good if thousands of civilians were killed this way?
From a military point of view civilian populations became an obvious target: right from the first days of war. People works in factories and make weapons, from the people enemy soldiers are drafted. In the first days of war the indiscriminate bombing of civilian areas was considered a crime but things changed very quickly. The city of Rotterdam was one of the first victims, on may the 14th, 1940. A threat of city bombing was used to force the dutch army's surrender; when the Germans tried to stop the airplanes, it was too late. The medieval city center was gutted by fire, hundreds of people died.
This was the beginning of "total war." Strategic bombing (that is, killing civilians and destroying homes) was used, quite famously, in the Battle of Britain, and after that used again in larger extent against the Axis countries, a majestic effort to destroy their war-making abilities.
Conventional air bombing is not a humane or charitable way of killing. Under a fire bombing, civilians die by burns, lack of oxygen, unbearable temperature, being slowly eaten by white phosphorus on their flesh, or buried under collapsed buindings. Both sides bombed civilians during the war, the Allied could make greater use of this way of killing because their air forces were stronger, and not because the other side was refraining from it.
Atomic bombings add radiations to the previous ways to die. Radiation exposure is particularly nasty because can kill you many years after the war has ended. But this was not really understood at the times. The atomic bomb was just another weapon to kill japanese quickly and efficiently (the USA air force was already doing that) with the additional advantage of showing quite clearly that resistance was futile. As for the "japs," the war had evolved from the "day of infamy" speech
to a grim butcher affair. American soldiers were quite unwilling to take
prisoners, and they imagined their enemies as cruel, untrustworthy
subhuman beings. This is the other side of the story everybody knows,
about japanese soldiers not surrendering because of honour and shame.
That's true, but even when they wanted to surrender they could not.
So, if you think that the military and political leaders of the USA spent sleepless nights reflecting on the moral drama of the atomic weapons and the righteousness of their use, you're probably wrong. There're other problems they were probably thinking about. Could the Bomb be a stern warning to the Russians, a way to make them restrain from further territorial ambitions (against Japan itself)? Could it be an admonishment to western friends too? Or maybe it was better not to show it to the world? In the end they decided to use it: peace talks were stalling,** and the Bomb could be a way to end the war.
When you read about the lives the Bomb spared, ending the war before a full fledged invasion of Japan became necessary, you read the truth. That is, worry about japanese lives was quite unlikely in those days, but for sure the USA worried about their soldiers. In the last island battle (Okinawa) they lost a great deal of men, so they could imagine a battle in Japan mainland could be even worse.***
So, the Bomb was just the logical development of modern war. There're a lot of reasons to harbor doubts about the "fighting the good fight" retoric, and the Allies having all the right on their side, in my opinion. Still, the most controversial military fact of the war is just a consequence of what happened before it, and probably it really spared a lot of lives.
(**) Main problem was, for the Japanese, not having guarantees about the emperor's fate.
(***) The hypotesis of using the Bomb on a uninhabitated target was considered, but probably such a demonstrative bombing could have been unimpressive if succesful, quite negative if the Bomb didn't explode.