Minor elaboration: Nagasaki was not an undamaged city. It had been bombed multiple times by conventional American bombs. But then Nagasaki was just a backup target anyway. Kokura was the primary target for the second bomb but it was obscured by clouds and smoke during the bombing run on August 9 so the plane went to Nagasaki instead. By the time they got there it too was socked in by clouds which is why the Nagasaki bomb fell so far off target.