The Japanese didnt plan to push into Siberia their plans where limited to the Maritime regions of Asia and Lake Baika.
The idea of launching an eastern offensive was first conceived at the time of the new plan of 1934. Three years later Japanese operational planning was again revised:
- At the outbreak of a war, the forces stationed in Manchuria will mount an offensive against the Soviet Union; reinforcements thereafter dispatched from homeland will join in the attack. This plan differed from its predecessor, which had called for an offensive only after the reinforcements had arrived in Manchuria.
- After successfully concluding the eastern offensive, Japanese forces should not immediately drive toward the Lake Baikal region, but should instead consolidate along the Hsingan Mountains Range and prepare for subsequent operations.
- Operational planning against must attach prime importance to actions against both China and the Soviet Union.
1) Look at a map. Lake Baykal IS far into inland. It's north of Mongolia, for God's sake, and Japan never managed to push that far inland in CHINA. And China has a couple of huge, navigable rivers, and the climate of paradise compared to Siberia.
2) The Stanovoy range would take ages to properly fortify... that place's almost as bad as the Urals, worse in some ways. And all this is contingent on Soviet Union co-operating with Japanese forces and letting them do so.
3) We've also been over this, there is really no earthly motive for the Japanese to attack Soviet Union... what do they hope to gain? To drive South, at least they can delude themselves into thinking they can gain oil, tin, rubber, all essential materiel of war. What to gain in Soviet Far East? Timber and coal, of which there is plenty in Manchuria, oil from Sakahalin, which she is getting by treaty ANYWAY? Empty space? Ice? Amur Cossack Host? What?
4) We've also discussed the likelihood of Soviet Union putting more importance in Vladivostok than Moscow... so no, it would be the historically present reservists, not the regular divisions, and even THAT is sufficient to hold off the Japanese
5) If we start the war in 1935 or something, as seems to be suggested, then things go even worse for Japan. Nazi German was in no position to intervene, and this was BEFORE the main purges...