[Torplexed]

I guess I've never thought very highly of this idea that a dual German-Japanese assault would have neccessarily toppled the Soviet Union.

It would not have helped Germany that much if the Japanese had invaded the Soviet Far East because besides the port city of Vladivostok the Japanese had no where to go and would have had no resources with which to drive further west. The mostly horse drawn Japanese Army, was not equipped or trained in Arctic warfare and would have perished in the Siberian Lowlands in the winter when it gets 70-120 degrees below zero! The Russians could have easily limited their advance with minimal forces in such an environment even with all of the Siberian divisions called away to defend Moscow. (As it was they didn't send all of them west!) And in the summer it's a hellish hive of bugs.

Eastern Siberia back then was a mostly trackless wilderness supplied by one slender rail line with no large cities or sources of supply, it is not suited for modern war in 1941, and even now would be a logistical nightmare to keep an army supplied in. Japan was ill-equipped to fight in the sub-Arctic as their reckless invasion of the Aleutians demonstrated. Plus, Japan badly needed oil and it wasn't until well after the war that a booming oil and gas industry was developed in Siberia. Those factories that Stalin shipped east during Operation Barbarossa were mostly resettled near the Urals. That's about 2,000 miles of endless swamps, forests and rivers from the Manchurian border.