A Soviet Agent by the name of Victor Sorge found out that the only way that Japan would have ever even considered attacking the USSR again, was only if Moscow fell. This information was passed down to Stalin who was then relieved as this allowed him to free up the reserves from the East.

Japan had nothing to gain from attacking the USSR. Japan needed oil rather then empty land which she would have encountered in the Siberia. Also if the Germans were suffering from the cold, how would the Japanese fair in Siberia, who were used to fighting in tropical climates? Not to mention the lack of tanks.

As for the US, I believe that she would have eventualy gotten involved in the conflict coming to aid of Great Britain.

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The war against Russia will be such that it cannot be conducted in a knightly fashion. This struggle is one of ideologies and racial differences and will have to be conducted with unprecedented, unmerciful and unrelenting harshness. -Adolf Hitler