This is a fantastic perspective on just how unprepared American intelligence was regarding what would be awaiting them in "Operation Downfall", which would begin with "Operation Olympic" on Nov. 1 of 1945, landing on Kyushu, and later "Operation Coronet", bringing another island assault starting March 1, 1946. Allied intelligence greatly underestimated their foe. The invasion would very well been a painstaking, brutal and lengthy war of attrition, and many of the Allied soldiers taking part came from the European campaign. This would be entirely different scale of fighting.
Initially, anyway, it was later estimated that the Japanese has a 3-2 advantage in troops, with 790,000 of mixed military background facing off against 550,000 Allied troops. There was also their air component:The battle for Japan, itself, would be won by what General Simon Bolivar Buckner had called on Okinawa "Prairie Dog Warfare." This type of fighting was almost unknown to the ground troops in Europe and the Mediterranean. It was peculiar only to the American soldiers and marines whose responsibility it had been to fight and destroy the Japanese on islands all over the south and central Pacific. "Prairie Dog Warfare" had been the story of Tarawa, of Saipan, of Iwo Jima, and Okinawa. "Prairie Dog Warfare" was a battle for yards, feet and sometimes even inches. It was a brutal and dangerous form of combat aimed at an underground, heavily fortified, nonretreating enemy. "Prairie Dog Warfare" would be what the invasion of Japan was all about.
The link posted reveals the myriad defenses that Japan had crafted in her defense, and Allied intelligence likely had no idea of much, if not most, of these defenses. It begs the question: Would a ground assault and invasion of Japan, lacking any atomic weapons, even been successful?The Japanese 5th Naval Air Fleet and the 6th Air Army had 58 more airfields on Korea, Western Honshu and Shikoku, which were also to be used for massive suicide attacks. Allied Intelligence had established that the Japanese had no more than 2,500 aircraft of which they guessed only 300 would be deployed in suicide attacks. However, in August of 1945, unknown to our intelligence, the Japanese still had 5,651 Army and 7,074 Navy aircraft, for a total of 12,725 planes of all types. During July alone, 1,131 new planes were built and almost 100 new underground aircraft plants were in various stages of construction.