What a lot of folks don't know is that the US had decent models for what the casualty rates of a battle would be, based on plenty of examples of the island hopping on the way to Japan, including Okinawa. The Japanese fighters did not give up, even when faced with overwhelming odds. The Japanese knew that invasion of the home islands was coming, and they had 600,000 troops stationed in the area where the US invasion with 400,000 troops was to take place. The fight for Japan would have been a hugely scaled up Okinawa. The estimate for the entire fight for Japan was that there would be 1 million US casualties and 10 million Japanese casualties. The way it happened instead, there were not only a lot less US casualties than that, but a lot less Japanese casualties than that.