You're welcome, and don't worry, I enjoy sharing knowledge.Thanks Bob. I know I've seen your analysis before and definitely didn't mean to make you dredge it all up again on demand. I wanted to reframe the discussion for all the people who thought there would be minimal effect on Soviet fighting in the west by looking at the fact that the east was still a net producer for them through the war. Your analysis makes that pretty clear. Beyond the acute loss in Lend-Lease while other routes are expanded the loss of hundreds of thousands of men, thousands of tanks and planes and tens of thousands of vehicles would surely be felt.
For the question of enticing Japan to attack, could the Germans basically promise them resources from the Soviet Union and equipment from Germany after the Soviet defeat? There was no shortage of optimism for either general staff. I could see an assessment that the two front war would force a Soviet surrender by 1942 with the transfer of oil, tanks and planes starting within months. I'm not saying this would be the outcome, but I could see both Germany and Japan believing it. As for whether Germany would be willing to give these concessions to Japan, with Stalin defeated they may still have to face the WAllies, but they should have some breathing room.
About the rest of your questions: even though the Soviet Far East produced some quantity of military equipment for that regime, it was not economically self-sufficient from European Russia. The Japanese army concluded that the relatively small population was disproportionately urban and consequently there weren't enough farmers in the region to grow food for everyone. Japanese calculations suggest that by 1945 yearly local production in Far Eastern Russia only covered 67% of bread-making grain, 66% of petroleum, and 38% of steel requirements. Hence, the balanced needed to be shipped from European Russia. Because the region's economy and the military forces stationed there both depended on the same Trans-Siberian railway, the more food, oil, and steel the Soviet regime shipped to the Far East, the less capacity would be available for military supplies in the event of war with Japan. Due to these limitations, Japanese planners estimated that the maximum size of any army force the Soviets could have fielded against them in the Far East at 55 to 60 divisions. (For comparison, the force the Soviets historically used in Manchuria in August 1945 - estimated by the Japanese at 1,600,000 men, 4,500 tanks, and 6,500 aircraft - was assessed as 47 divisions.) Further information can be found in JSOM vol. XIII (one of the 'special studies' prepared by the Japanese for the US Army after the war) where this subject is discussed in detail from page 17 onward.
In my opinion, the moment where the Japanese might have been convinced to collaborate with Germany in an invasion of the USSR was Matsuoka's visit to Berlin in March 1941. The Nazi foreign Minister Joachim von Ribbentrop deliberately concealed the details of operation Barbarossa from the Japanese and instead advocated for an attack on Singapore. Then the next month Matsuoka went to Moscow and concluded Japan's neutrality pact with the Soviets; after the Nazis and their European allies invaded the USSR in June Matsuoka then turned around and became one of the main advocates for tearing up the treaty he himself collaborated on and attacking the Soviets. Many Japanese army officers wanted the same, but the navy advocated only partial preparations (inadequate for an attack 'at a moment's notice') so that any buildup in Manchuria would not interfere with the other aggressive plans against Southeast Asia. The Japanese army itself was not unanimous on the extent of military preparations against the USSR, and the overall commander in China, Shunroku Hata, strenuously objected to the large redeployments of troops then under his command that the 'strike north' proponents wanted. General Itagaki, the incoming Korea Army commander, also opposed large-scale redeployments of troops (IIRC beyond 1-2 additional divisions) to Manchuria.
Even though the Japanese army was divided and the Japanese navy generally opposed attacking the USSR, the "strike north" faction, led by General Shinichi Tanaka, ended up getting their way when it came to the size of the military buildup in Manchuria - on the condition that no actual attack would take place without Hirohito's authorization. However, when the Germans started to bog down in Europe and the Soviets failed to transfer half their Far Eastern forces westward the Japanese army started to consider the Kantokuen plan to be less attractive. The final nail in the coffin came on 1 August 1941 when the United States cut off all oil exports to Japan, followed by the Netherlands East Indies shortly thereafter. After this, the Japanese military formally decided on 9 August to concentrate on plans for an attack against Southeast Asia and the buildup in Manchuria and Korea was limited to only 16 divisions.
- Japan's regime felt the Molotov-Ribbentrop pact was a 'betrayal' of the anti-Comintern pact
- Japan's regime concluded their own neutrality pact with the USSR in April 1941
(^These two tended to divert the attention of the Japanese militarists to the south)
- Germany attacked the USSR in June 1941; the Japanese regime felt "betrayed" again
- The Japanese militarists were divided between focusing on the south (preferred by the navy and some army elements) or building up for an attack on the USSR (preferred by some army elements. The Japanese army was arguably more in favor of attacking the USSR than going along with the navy's plans)
- The Japanese war ministry decided on a halfway measure: some additional forces would be sent to Manchuria, but not enough to undermine the navy's plan for aggression to the south.
- The buildup in Manchuria historically reached the level preferred by the war ministry, and measures were in place to increase the number further, but outside events prevented this (such as opposition from Hata and the US/NEI oil embargo).
- Thus, the Kantokuen plan was stopped before reaching the levels advocated by General Tanaka and others because the Japanese regime decided to concentrate on invading Southeast Asia first.
- The Japanese didn't completely abandon the idea of invading Siberia in 1942, but because of several factors this obviously never happened.