;^)
The old town was largely made of wood, so the ignition of exposed surfaces facing the kaboom was widespread, and led to a conflagration that followed the chimney effect of the blast. One bomb, one city, done.
And prior to that, allied incendiary bombing had already destroyed 45 Japanese cities.
Despite all that, it took *a second nuke* to convince them to surrender.
https://freerepublic.com/focus/chat/4121786/posts
https://freerepublic.com/focus/chat/3191014/posts
https://freerepublic.com/focus/news/3325241/posts