Hersey is careful not to do any direct moralizing on the issue of atomic warfare. Professionally, he was journalist, not a novelist. The book is primarily concerned with showing the effects of the bomb on six different people who survived it. However, his choice of details and story lines unequivocally shows that he is opposed to the use of the bomb.
It would be difficult to write something like this with a rounded presentation. You would need to show how the bomb ended the war earlier, thus saving many thousands of American lives that would have been lost in an invasion of Japan.
The fact that Hersey is writing a book about those who survived the Holocaust represents how opposed he is to atomic warfare. Consider his writing about the destruction of Miss Sasaki's office building:
There, in the tin factory, in the first moment of the atomic age, a human being was crushed by books.
Consider his viewpoint in this comment that knowledge from books, scientific knowledge, is the net result of the atomic bomb, a weapon or "device" of destruction. In articulating the themes of survival and sacrifice, Hersey puts a human face on the human target. At a time when the American military and political leadership wanted to make the argument that the destruction of Hiroshima and Nagasaki was made out of military concern, Hersey rebukes that with the human account of this decision. In narrating their tales of survival, Hersey speaks out against the use of nuclear warfare and atomic energy for destructive ends. The stories of these survivors and how some committed their lives to peace and sacrifice, while others never fully recovered from radiation is his contribution to how atomic warfare is fundamentally destructive.