What is the central idea of the bombing of Hiroshima

After the atomic bomb kills 100,000 in Hiroshima, the six main characters of the book wonder why they survived while so many others perished. They reflect that it was small, unconscious, and seemingly coincidental actions that spared their lives at the moment of impact.

The central idea was to eliminate japan into submission so that they must surrender. They wanted the bomb to be so massive that their is no choice but for japan to surrender.