Look what happened in 1918-1920 ( Red Vs White) This was the allied attempt to fight the Reds in Siberia
http://en.wikipedia.org/wiki/Polar_Bear_Expedition
“However, these two fronts each became hundreds of miles long and were extremely narrow and difficult to supply, maintain and protect. By the end of October 1918, they were no longer able to maintain the offensive and acknowledging their fragile situation and the rapid onset of winter, the Allies began to adopt a defensive posture.”
We already did this and failed too. Its like invading Antarctica, except its thousands of miles of tundra and wilderness. Nothing can support an army over such distances without infrastructure. The problem is the territory in the far east is just as bad and farther from support.
Its kinda the same reason why Germany or Italy didn’t bother to fight in the Sahara proper and just fought along the coastline and never used the option of sending armies south to hook underneath. Its just not possible to sustain large armies. The Russians could easily bottleneck the enemy at choke-points where they took an easy route and made the ‘invasion’ a 50 year protracted event.