"I will not be neutral as between the fire and the firemen."
However in my personal opinion, even today, after 50 years of loosening, Japanese culture is very tightly organized, and civilians there could easily organized into an effective fight force - including men, women and children. 100 million of them.
It is not unreasonable to believe that (especially the battles for Iwo Jima and Okinawa), that millions of Japanese civilians would have fought an invasion of the Japanese main land. The Japanese were already using suicide bombers (you know them as Kamikaze pilots) towards the end of the war. Instruction of civilians as human bombs was beginning in anticipation of invasion when th e war ended.
I love Japan, and the Japanese.
I honestly don't believe that there would be a Japan as a country today, if not for the atomic bomb. It would still be a battlefield.