Ok, let's consider the facts of the war for a second.
The prolonged fire bombing of Tokyo killed more people than both of the nuclear attacks put together.
The Japanese were given the opportunity to surrender after we dropped the first bomb, but refused.
Germany already knew that the potential for the bomb existed. In fact, Germany actually had plans to create the bomb, but put the project by the side because they did not believe that the project would be completed before the war was over.
The only reason the Japanese surrendered was because we dropped the second bomb, proving that we not only had the weapon, but that we could reproduce it as well.
Before the power of the nuclear weapon was demonstrated to Japan, they had no reason to believe that we had succeeded in building one. Would you have believed Japan if they warned us that they had succeeded in building one?
Yes, we knew that the bomb would be destructive. That's the whole reason we built the damned thing. Did we know the full potential of the weapon? No. No one did. Einstein himself didn't know how powerful the thing would be, although he did fear that we were likely underestimating its potential. But no one knew how powerful the weapon was until it was unleashed. It was an exercise in experimental physics. It was far more destructive than we had anticipated. Does that excuse what we did? No. In fact, nothing does. Yes, we killed thousands of innocents. I'm not proud of that, nor will I ever be. We also probably saved the lives of millions. I hate having to reduce sentient life done to numbers and balance them on a scale, but I'm afraid that sometimes it's the only choice we have.
Oh, and Vivi22 was correct. Scientists working on the project to develop the Fusion Bomb were at one point worried that the bomb would ignite the atmosphere, completely destroying it and eventually eliminating all life on this planet. As it was, when the Bravo Blast came around (the code name for the first Hydrogen Bomb detonation), it was still beyond the projections that we were given for the probable destructive power of the blast.
