Not only was Truman told of the atrocious casualty projections but the Joint Chiefs forcast a ferocious war for the conquest of Japan that would’ve taken into the 1970’s to accomplish.My USMC Dad told me that The choices were thousands of casualties or millions of casualties,there was never any third option.
My dad was training to fight in the mountains of Japan and he was told that it would be very difficult to survive.
One of the factors that they took into account was the ferocity of the Japanese forces in Iwo Jima and Okinawa. Very few prisoners were taken as they chose to fight to the death and it would have been worse on the Japanese homeland.
It would have been truly awful if they had gone on with that plan.