The real argument against dropping the bombs isn't whether or not it would be the best and most humane way to end the war with Japan, it's about the future ramifications of showing the entire world that the Atomic bomb was a viable military weapon. Even this, however may not have been entirely negative, but it has yet to play out completely. The development of nuclear weapons may have been responsible for the lack of a World War 3, without the risk of MAD the Soviets and the west may well have resorted to more traditional warfare. Without the need for missiles to strike anywhere on the planet space exploration likely wouldn't have advanced as quickly, or at all. On the other hand if America hadn't used them, the atomic weapon might have stayed a piece of science fiction, and we wouldn't still have the threat of total annihilation looming over our heads. I could probably write a term paper on the far-reaching ramifications of the decision to drop the bombs or not, but I think I've made a clear point that the immediate war wasn't the only factor to consider.
As for dropping one of the bombs as a demonstration, there are valid arguments against it, but the most likely reason it didn't happen is less valid. There was actually a petition signed by nearly all the scientists and technicians that developed the bomb asking the President to demonstrate the weapon in an uninhabited area, but certain military men made sure it never reached him, so it was never really given any consideration.
Edit: Oh yeah, what really convinced Truman to drop the bomb was one of his advisors saying "What will future generations say when they learn that you could have ended this war, but chose not to?". I may have paraphrased that a little but it's a powerful thought.
As for dropping one of the bombs as a demonstration, there are valid arguments against it, but the most likely reason it didn't happen is less valid. There was actually a petition signed by nearly all the scientists and technicians that developed the bomb asking the President to demonstrate the weapon in an uninhabited area, but certain military men made sure it never reached him, so it was never really given any consideration.
Edit: Oh yeah, what really convinced Truman to drop the bomb was one of his advisors saying "What will future generations say when they learn that you could have ended this war, but chose not to?". I may have paraphrased that a little but it's a powerful thought.