I've always been split. On the one hand, I feel that the distinct tactical disadvantage we were in with Japan because of location made them necessary to make the war end. I also think that the bombings were a big gamble, as there is no real way to quantify the difficulty that we would have had just invading the country (as Speedy Samurai said, Japan was considering surrendering), so world opinion could have turned quickly if the rest of the world found the ends didn't justify the means (and it was becoming obvious that the Cold War was looming far before WWII ended, so that could have been a bad thing).
On the other hand, one could look to the Western Theatre to show that the bombing of Hiroshima was unnecessarily cruel (though it would be arguably impossible to determine this beforehand, it could have at least prevented Nagasaki). The firebombings of Dresden, for example, showed that a single bombing of a populated city would have a devastating effect on morale (indeed, Dresden claimed roughly the same amount of lives as the atomic bombings did combined, according to some). At the time, too, German pride and resolve for the war effort was easily far higher than Japans was a year later (when Japan was the one left holding the ball with Italy and Germany both defeated), and I am sure Truman knew this. So one could make a fine argument to say that an attack of that caliber really wasn't necessary (and Nagasaki was pure overkill to the point of War Crimes, in my opinion) simply because it had been proven that a nation with far stronger resolve was dealt a huge blow by similar proceedings, and that Japan (who was considering surrender) was no where near as dedicated to the war as Germany as to necessitate such an attack. Furthermore, the act was carried out seemingly just to get an unconditional surrender rather than the surrender being thrown around Japan's inner circles.
Nagasaki is an interesting situation too. I personally believe that it was an immoral act, mostly because of its reasoning. It was said to be necessary to completely destroy the Japanese resolve as Hiroshima could not (as if Japan was daring us to do it again), but I think that is a falsehood. The quick timing between the blasts led to Nagasaki being bombed before Japan really even knew what had happened at Hiroshima. Its as if the U.S. bombed Nagasaki accepting the demands of the U.S., without even thinking that maybe Japan didn't even know what had happened in the first place (like punishing them because they had been punished). I think the U.S. should have taken a couple of different options:
- Given Japan more time to figure out what had actually happened at Hiroshima so they knew what they were dealing with. This could have spared countless lives, as Japan may very well have surrendered when they learned what had really happened there without any invasion or second bombing at all. Indeed, warnings of the Nagasaki bombings didn't even arrive until the day after the bombing (of Nagasaki) actually happened, when even a week of warning after Hiroshima may not have been enough with the panic in the Japanese government.
- Supplanted the initial bombing with an invasion force; to take the surprised, confused and panicked Japanese of guard and have a quick victory. I doubt the loss of life in this case would have been anywhere near as much, even with both sides combined. With a quick invasion during the panic of Hiroshima, lives lost on both sides probably would have been far lower if the Japanese government didn't surrender immediately. Hell, even without the Hiroshima bombings, I doubt total lives lost would have been near that of the bombings by themselves (remember, Operation Overlord and Operation Downfall would have taken place under completely different circumstances, as Japan would have had far more places to be invaded from and therefore would have been spread far thinner than German forces at Normandy).
There are problems with both of these solutions, naturally (#1 assumes that the bombings wouldn't strengthen Japanese resolve, which could have happened, and #2 would have had more American lives lost at a time where war weariness was starting to take America), but I hardly think that Nagasaki was the answer to the equation.
To sum up? I guess I would say the Hiroshima was necessary depending on how you looked upon it. If America felt that the Japanese surrender would have led to further aggression (and a potential WWIII), than I think one could argue that it was necessary. If one thought that the mere threat of a repeat of Dresden would have shaken Japanese officials enough to surrender, or that a conventional invasion would have had a far lower life toll; then I think you could argue it was unnecessary. In either situation, however, Nagasaki was pure criminal. Japan wasn't given enough time to assess what had actually happened at Hiroshima to react to U.S. demands regardless, but a conventional invasion could have at that point been easily done and may not have even been needed.