I often hear the claim that Stalin pulling out the Siberian divisions from the Manchurian border in 1941 in order to defend Moscow was the decisive factor that killed Operation Barbarossa, and that if the Japanese and Germans cooperated a joint invasion of the USSR the Japanese could have tied those divisions down in Siberia, leaving the road to Moscow far less defended, so much so that the Germans would likely have taken it. Leaving aside the issue of whether the Japanese would even agree to a war with the Soviets in '41, this raises two questions:
1. Since the Japanese were tied down in a war of their own in China and quickly running out of the resources necessary to sustain it, could they have even mustered the force necessary to tie down all those divisions in Siberia, without dangerously straining their resources or their jeopardizing their ongoing war effort in China?
2. Did all those divisions make such a big difference, really, in Moscow? Could the Germans have won there if only those extra Soviet men weren't there?