Would a D-Day like invasion of mainland Japan been feasible if you had the Soviets involved?

Assuming the Atom bombs were delayed by months if not more in this time line;

a) Would the equivalent of an Operation Overlord in Japan been feasible with the support of the Soviets invading themselves from the North perhaps?

B) Would the WAllies actually consider a joint operation with the Soviets or do anything to go at it alone?
 
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They might be able to do some minor landings on Hokkaido but they would have to take the rest of Sakhalin Island first. Then they would have to deal w/ supporting the operation and sustained logistics was never a Red Army strong suite.
 
The allies had no intention of including the USSR in the invasion of mainland Japan, and the Soviets lacked the ability to offer a meaningful contribution in the near future.
 
Same answer. The Soviets were focused still on swallowing up Europe. They would not have been in position to invade the Japanese home islands. While they, (Soviets) could have moved everything needed for a Japanese home island invasion East, it is unlikely they would have done so. Perhaps a token symbolic effort. But perhaps they would have occupied more of China.
 
Answering the first question, I believe the actual planned invasions of Japan were both bigger than the Overlord invasion.
 
I believe significantly so as well. Which is why I wonder if the Soviets could have alleviated that burden somewhat?

The Soviets would have required allied assistance to make any landings and consequently would have increased the logistical burden rather than lessened it. The US even denied the Soviets naval support in the Kurils during the run-up to the USSR's declaration of war on Japan on the grounds that the Americans already had their hands full.
 

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The Soviets would have required allied assistance to make any landings and consequently would have increased the logistical burden rather than lessened it. The US even denied the Soviets naval support in the Kurils during the run-up to the USSR's declaration of war on Japan on the grounds that the Americans already had their hands full.

Project Hula says hello: https://en.wikipedia.org/wiki/Project_Hula
 
While limited amphibious lift was given to the Soviets, even if they made a serious landing on Hokkaido, and given the sea and air conditions when OLYMPIC was supposed to go off this would be iffy, it would not help the USA at all. Basically to make any serious assault the Soviets would need a lot of specific LL from the USA. They had ZERO experience in any amphibious assaults over open ocean, the assaults they did had been very short range and basically coastwise. A Soviet occupation of all of Hokkaido could not have occurred in time to even theoretically aid OLYMPIC. If the Soviets allowed the USA to establish some air bases in Southern Hokkaido, that might have been useful if CORONET became necessary. The odds of the Soviets allowing such US air bases, operating under US control for US defined missions closely approaches zero. Note the Soviet attitude towards any US/Allied air activity based out of the USSR against Germany.
 
For starters, there was already on Overlord equivalent planned: Downfall, which was actually to be the bigger amphibious invasion of the two. Much discussion has been had over the years of what the Soviets might do, and general consensus is that maybe they could land in Hokkaido. However, this wouldn’t actually help Downfall succeed, for three simple reasons:

1. It’s very far away from the Allied landings, which were to be in southern Kyushu and the Kantō region of Honshu, and far away from most of the remaining Japanese troops.
2. The Soviets aren’t likely to get any further.
3. The Japanese are also in no logistical position to shift many troops north.

So a Soviet landing won’t intersect many Japanese troops and is very unlikely to draw any away from the Overlord landings.
 
As others have said, it was just not practicable to involve the Soviets in an invasion of Japan even if the US had needed or wanted their assistance. There just wasn't enough shipping to relocate troops coming from Europe back to the US or directly to the Far East, to stockpile equipment for the invasion much less using any to move Soviet troops from Vladivostok to the invasion staging points in the Philippines or Okinawa. A Soviet invasion of Hokkaido wouldn't do much to the strategic balance. The Japanese knew where the main landings were going to be and reinforced those areas. A Soviet invasion of Hokkaido, if successful which is doubtful, would have only tied about 2 IJA division equivalents; not much in the scheme of things.

As far as size, Operation Olympic, the invasion of Kyushu, was going to be about twice the size of Operation Overlord. Olympic would have 9 divisions in the invasion first wave with another 5-6 divisions in immediate reserve and a reinforced division making the pre-invasion landings on offshore islands. Overlord had only 5 division in the first wave plus 2+ airborne division equivalents. The number of supporting warships, troopships, cargo ships and other logistics support was gigantic in comparison to Overlord, primarily due to the distances from the staging areas to the invasion location.
 
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