By the year 1939, Japan's ambitions were too big to keep it in check. The Empire was now more hungrier than ever for territorial gains and it's ambitions knew no bounds. This can be observed with their invasions of Manchuria (1937), Formosa (Taiwan) (1910) and the Korean Peninsula (1910). 

 

While they were always ambitious about uniting and ridding the far East from colonial rule, they always considered their northern border with the USSR an impending and perpetual cesspool of both territorial ambition and threat to national security.

 

Having a direct border with the USSR would essentially always keep Japan's ambitions in check, with Soviet airfields, naval dockyards and artillery units all aimed and ready to strike major japanese cities within a matter of hours, which just won't sit well with the Taichos in Tokyo. 

 

Which makes it all the more imperative to ensure that their northern flank is secured.

 

Historically it was evident that Japanese chose not to focus or delve into a conflict with the USSR and instead focused it's efforts on creating the 'Greater East Asia Co Prosperity Sphere' and designating the United States as it's principal rivals.

 

This time, the Emperor Hirohito himself has ordered his generals to not get complacent and divert their attention from the South-East and now instead focus their complete efforts on territorial expansions into the vast Siberian Tundra, thereby securing the Empire from a potential surprise attack on the northern flank.

 

No one dares to disobey his orders. Only time will tell how successful they will be.