What is being missed in much of this discussion though is that a Soviet invasion of Japan is not going to occur with everything else remaining the same as in OTL. For a Soviet invasion to occur it must mean that:
- the Soviets entered the war in the Pacific sooner or with more forces available so that by August 14 they are already on Hokkaido. Perhaps due to the conclusion of the war in Europe happening faster (perhaps the Soviets take Berlin in February 1945 allowing the war to end 2-3 months earlier in Europe and allowing the Soviets to enter the war in the Pacific in June 1945 or to build up even more forces by August 1945)
OR
- the Japanese do not surrender on August 14/15 as per OTL. If this happens then Truman is not going to oppose a Soviet landing on Hokkaido. Remember that in 1945 Truman's primary aim was to bring about the surrender of Japan as quickly as possible and with as little loss of life as possible (i.e. as little loss of life generally for all combatants but especially for American servicemen). His secondary aim was to limit Soviet influence in Asia. There is no possible way Truman is going to sacrifice his primary aim for his secondary aim. It will only be the other way around. This doesn't mean he would agree to a carte blanche for the Soviets in Japan, but that if the Soviets were in a position to land in Hokkaido after August 15, 1945 and with NO Japanese surrender in sight then he would accept it. He might then take issue with how much territory the JCS had proposed to be given to the Soviets in their zonal map (he would probably recommend that northern Honshu be removed from any Soviet zone and be placed in the American zone, leaving only Hokkaido for the Soviet zone), but he can't in all seriousness oppose a Soviet invasion under those circumstances. If he did the Republicans and his own Democrat Congressional leaders (and basically many a Democratic Congressman) would be calling for his head - they would have to be explaining to their voters just why it was that the President was refusing to allow America's allies (and in 1945 most of America's population had fed a steady stream of government messages (or propaganda) about the USSR and UK being firm allies...heck even Churchill's 1946 "Sinews of Peace" speech (his "iron curtain" speech) was not well received at the time in the US as many Americans still considered the USSR an ally) to carry out measures to end the war early and bring their sons, brothers and fathers home.
Now even if MacArthur still didn't like it then that is MacArthur's tough luck. MacArthur did not have voters to answer to directly about why a particular policy was being chosen which might lead to the unnecessary deaths of soldiers related in some way to those voters. But MacArthur was ultimately answerable to Truman who in turn was answerable to such voters. And Truman will know that no matter how much MacArthur and he might disagree with a Soviet invasion that in the long run it might bring the war to an end sooner AND that in the end MacArthur only had 1 vote, whereas the multitude of people back home who are related to the soldiers that he was going to send into harm's way had many more votes due to sheer numbers. Truman then would have most likely chosen a middle route of not opposing a Soviet invasion but at the same time attempting to limit whatever political gains the Soviets might derive from such an event (while at the same time trying to maximize the military advantage America could gain from such an event - for instance a Soviet invasion might cause the Japanese to delay sending more reinforcements to Hokkaido and the Kanto Plain...and a Soviet liberation of Korea might allow for new airbase and naval facilities to be built in southeastern Korea and in Hokkaido which could be used to aid in the invasion of Kyushu and the Kanto Plain).
- the Soviets entered the war in the Pacific sooner or with more forces available so that by August 14 they are already on Hokkaido. Perhaps due to the conclusion of the war in Europe happening faster (perhaps the Soviets take Berlin in February 1945 allowing the war to end 2-3 months earlier in Europe and allowing the Soviets to enter the war in the Pacific in June 1945 or to build up even more forces by August 1945)
OR
- the Japanese do not surrender on August 14/15 as per OTL. If this happens then Truman is not going to oppose a Soviet landing on Hokkaido. Remember that in 1945 Truman's primary aim was to bring about the surrender of Japan as quickly as possible and with as little loss of life as possible (i.e. as little loss of life generally for all combatants but especially for American servicemen). His secondary aim was to limit Soviet influence in Asia. There is no possible way Truman is going to sacrifice his primary aim for his secondary aim. It will only be the other way around. This doesn't mean he would agree to a carte blanche for the Soviets in Japan, but that if the Soviets were in a position to land in Hokkaido after August 15, 1945 and with NO Japanese surrender in sight then he would accept it. He might then take issue with how much territory the JCS had proposed to be given to the Soviets in their zonal map (he would probably recommend that northern Honshu be removed from any Soviet zone and be placed in the American zone, leaving only Hokkaido for the Soviet zone), but he can't in all seriousness oppose a Soviet invasion under those circumstances. If he did the Republicans and his own Democrat Congressional leaders (and basically many a Democratic Congressman) would be calling for his head - they would have to be explaining to their voters just why it was that the President was refusing to allow America's allies (and in 1945 most of America's population had fed a steady stream of government messages (or propaganda) about the USSR and UK being firm allies...heck even Churchill's 1946 "Sinews of Peace" speech (his "iron curtain" speech) was not well received at the time in the US as many Americans still considered the USSR an ally) to carry out measures to end the war early and bring their sons, brothers and fathers home.
Now even if MacArthur still didn't like it then that is MacArthur's tough luck. MacArthur did not have voters to answer to directly about why a particular policy was being chosen which might lead to the unnecessary deaths of soldiers related in some way to those voters. But MacArthur was ultimately answerable to Truman who in turn was answerable to such voters. And Truman will know that no matter how much MacArthur and he might disagree with a Soviet invasion that in the long run it might bring the war to an end sooner AND that in the end MacArthur only had 1 vote, whereas the multitude of people back home who are related to the soldiers that he was going to send into harm's way had many more votes due to sheer numbers. Truman then would have most likely chosen a middle route of not opposing a Soviet invasion but at the same time attempting to limit whatever political gains the Soviets might derive from such an event (while at the same time trying to maximize the military advantage America could gain from such an event - for instance a Soviet invasion might cause the Japanese to delay sending more reinforcements to Hokkaido and the Kanto Plain...and a Soviet liberation of Korea might allow for new airbase and naval facilities to be built in southeastern Korea and in Hokkaido which could be used to aid in the invasion of Kyushu and the Kanto Plain).