If they were willing or not, the experiences of Okinawa and Saipan revealed that there would be no "non-combatants" in any invasion of the home islands.
Then there is the idea of a blockade. Is starving non-combatants to the point that they overturn the military leadership to end the war attacking "civilians?"
Interesting. But guess you are assuming that ground troops would be met by civilians with various relative ineffective weapons. That still does not justify mass killing of elderly and children with devastating nuclear bombs. Use of force must be proportional to the force trying to eliminate.
Really hard to believe that America could not find less devastating way to handle such a threat.