When Hersey published this novel, the events of Hiroshima were still fresh in everyone's minds, but Americans did not realize yet that they really had no idea what happened in Hiroshima. Americans saw the bombing as a necessary evil and believed that this was a last resort. Hersey wasn't commenting on that specifically, but he was revealing a human side to the tragedy. Hiroshima was a place where civilians lived, and they are the victims of a terrifying strategy. He recognized that Americans justified the bombings too much and needed to see the complexity of the issue. This book helped Americans recognize the effects of the bombing on Japanese, physically and emotionally-- especially the long term effects. He was able to do this without demonizing America, which helped. A world where "necessary" evils of this proportion are devastating to the entire society, not just the victim.
At least one more reason Hersey wrote the book was to recognize Japanese moral character and show how they grew from the tragedy. The story follows six characters who show tremendous courage, charity, and strength.
As far as evidence is concerned in supporting these ideas, you will want to look for events in the novel that show character strength in the tragedy. Mr. Tanimoto is especially important because he becomes some sort of hero in American eyes, but fails to bring recognizance to American involvement in the rebuilding for peace.
Hiroshima is not a novel.