Okay, so let me again preface this by saying my statement is based on the U.S. and Japan being in the same position as they were in August of 1945. Obviously I get that, realistically, there will be differences compared to OTL given the changed nature of the war, but that opens up so many possibilities as to make overall speculation impossible. Honestly, too, this is being generous to the United States by assuming the Pacific War is in the same state as OTL by August of 1945, given how much tougher the Germans will be; the only way I can see the war having gone worse for the Japanese is if the U.S. elects to take a Japan First strategy given the situation in Europe, with all other circumstances being generally favorable to Japan. Case in point, if we're sending the B-29s to Europe, they aren't there to be bombing Japan, etc. With that stated,
@BobTheBarbarian did the research on this and
stated the case far more eloquently than I ever could in this thread. I'll provide a tl;dr verision, however.
The situation of mid to late-1945 needs to be viewed through the prism of the intelligence both sides were working on. In this regard, the Japanese were completely dominant over the United States, having correctly figured out the timing, sequence and even invasion points the Americans were going to use as part of Operation DOWNFALL in Kyushu. The Japanese had deduced the entire planning of DOWNFALL according to D.M. Giangreco's Hell To Pay, with their only error being, if it could be even called that, their assumption the Allies were going to deploy more forces than they actually were planning for. They expected the United States and its allies to commit almost 10,000 plans when in reality it would've been 5,000 or less, while they also estimated they (Allies) would land between 15 to 40 divisions; in reality, the Allies were planning for 14. The Japanese were preparing to meet and defeat a much larger force than the Allies could actually bring to the table.
On the flip side, the U.S. was completely off. Case in point was MacArthur's G-2 estimating that, by X-Day on November 1 in 1945, the Japanese in all of Kyushu would have 300,000 men and 200 tanks. In actuality, the 57th Army based in southeast Kyushu alone had that amount of strength. When the 700,000 Marines and U.S. Army soldiers of the 6th Army landed in that November, they would've been facing in total 900,000 Japanese soldiers dug in with permanent emplacements just like at Iwo and Okinawa. Japanese planning as part of KETSU-GO also envisioned that, immediately following the American invasion, an addition 90,000 troops in four divisions were to be transported across the narrow waterway of the Shimonoseki Strait. IGHQ projections had the entire force outfitted and ready by October of 1945, with the logistical underpinning of the force being six months worth of supplies that had been stockpiled, ready for the decisive battle. Outside of the immediate invasion beaches, no firepower advantage would've existed for the Americans; Kyushu is extremely mountainous and thus the ranges involved would've produced a situation the U.S. found like that in Italy or Korea, with close in fighting at very short ranges. In essence the U.S. would've found itself launching frontal assaults against an enemy that already outnumbered it. By now I've belabored the point that by 1945 the Japanese had figured out the tactics and strategy needed to inflict 1:1 losses on their enemies, but the point needs to be re-stated here because at 990,000 to 600,000 the basic math is clear how this would go.
Finally, and most importantly, was the air campaign. IGHQ had been stockpiling aviation for months in preparation for the invasion, with total inventory being 1,156,000 barrels by July of 1942. Much the same had been done for pilots, with IJA having 2,000 pilots with at least 70 hours of flying time while the IJN had 4,200 on hand who were considered sufficiently trained for night or low light missions; given the type of challenges those conditions presented, that means they were well trained. Overall, when the Japanese formulated KETSU-GO starting in July of 1945, the plan called for 9,000 aircraft to be brought to bare against the invasion fleet. Contemporary to this, the Japanese inventory already contained 8,500 ready planes and IGHQ expected another 2,000 by the fall. When the Allies conducted a census in August following the surrender they found 12,684 aircraft of all types in Japan, suggesting that IGHQ's estimates were spot on for 10,500 aircraft by November. As for planned uses, of the 9,000 to be used in KETSU-GO, kamikazes were to comprise 6,225 of the total.
That last bit is perhaps the most important, as experience at Okinawa had shown that a 6:1 ratio existed in the expenditure of kamikazes to achieve a successful ship sinking. Japanese planning held, and U.S. estimates agree with them, that they believed in the initial 10 days of the invasion they could sink at least 500 transports out of the expected 1,000 the U.S. was bringing for the attack. This would've amount to the loss of about five divisions and much of the logistical network, crippling the invasion before it even stormed the beaches. There is every reason to believe this would've worked, as the Japanese would've enjoyed several advantages they didn't have at Okinawa, such as:
- The mountainous terrain meant that Japanese attacking aircraft would've been shielded from radar detection almost until they were right up on the fleet. At Okinawa, the U.S. had been able to deploy destroyers as pickets dozens of miles out but that wouldn't have possible here because the invasion fleet obviously had to be closely anchored off Japan.
- The "Big Blue Blanket", which was an Anti-Kamikaze tactic devised by the U.S. during Okinawa, involved masses of fighters kept aloft and being fed data by the picket ships. However, this would've been impossible to counter the Japanese here, as the U.S. was only bringing 5,000 total aircraft from the Far Eastern Air Force in the Ryukyus and the carriers of the 3rd and 5th Fleets. The problem, as outlined by Giangreco, was that U.S. planning called for TF-58 with its 1,900 plans to be 600 miles to the North attacking targets in Honshu instead of supporting the 7th Fleet. This left just two carrier groups to provide a combat air patrol for the fleet, which means that American fighters would've been outnumbered by the Japanese by about a staggering 10 to 1. In other words, even if every American fighter pilot became an ace during those first 10 days, thousands of Japanese aircraft would've still broken through.
- The Japanese had 60 airfields on Okinawa and the aforementioned fact of short distances to target meant that mechanical issues, a problem that plagued kamikaze operations during Okinawa given the hundreds of miles distance from Japan to the island, would not have been anywhere near as prevalent.
