Post-war inspections found that the Japanese were stashing aircraft and weapons for a planned invasion - more so than what the US thought they would have. Further, they were training their civilians for resistance fighting.
The second point is important. If, in that time, non-uniformed fighters made a habit of hiding among legitimate noncombatants to launch attacks, the US forces would have killed the bad guy and anybody else around them. It is not too hard to imagine that the Japanese people would have been slaughtered in numbers that would have far exceeded the dead from Hiroshima or Nagasaki.
Good point.