How do you explain Downfall and the US mobilization plans involving over 60 armour divisions or the many of thousands of landing craft the US built by wars end? It is not a question of if the US invades North Africa or Europe only when and if it is successful.
Because it was against Japan, which had about 3.5% of the total global industrial potential in 1940, compared to the U.S. at 41.7%. Even taking into consideration the damage done to the occupied European countries, the Reich controls, ATL, around 36% of the glbal 1940 industrial production
Because Japan had, even in September of 1945, more than 1.8 million troops, and the vast majority of its heavy equipment, on the Asian mainland, and several hundred thousand other troops trapped in the PI, on Truk, and on other islands that had been isolated and left to wither on the vine.
Because Japan's industry, such as it was, had been stared for fuel, raw materials, and electrical power by the hugely successful U.S. submarine and naval mining campaign. This same campaign had greatly reduced the daily caloric intake of the average Japanese subject.
Because the U.S. had a massive logistical jumping off point 350 miles from Kyushu across waters that were effectively an American lake, and a second, equally huge logistical base 1,000 miles from Kyushu.
You will note that, unlike Japan, the Reich has sufficient oil, an embarrassment of raw material riches, sufficient food, and the ability to move forces, by rail, across interior lines of communication. In Planning any landing Europe the U.S. has one logistical base within 1,000 miles of the continent (the aptly named Iceland) until the Faroes and Shetlands are captured (both of these targets are well inside land based air out of Scotland and Norway).
It is also worth noting that, unlike IOTL, any engagement against the Reich is purely voluntary on the part of the United States. There is no plucky London standing firm in the face of the Luftwaffe, no Churchill making his "Never Surrender" speeches (clearly ATL, the British DID surrender). What is there? The sort of invasion that boggles the mind. American planners (except MacArthur, who refused to even look at the intel if it didn't agree with him) expected to take more KIA in the first week on, and in the waters surrounding, Kyushu than the U.S. had absorbed during the ENTIRE PACIFIC WAR. Compared to the proposed invasion in this ATL, Downfall would have been an administrative landing.