>>512994872
Japan was defeated in Khalkin Gol before the Molotv-Ribbentrop pact, the pact actually stopped the soviet advance into Manchuria.
I have no idea why Japan had such a hardon for Siberia, they even tried to invade it during the war between the red and the whites during the communist revolution, but were stopped because the japanese emperor didn't trust his generals to be to settle and start nation building in Russia without his direct supervision.
This was about the same time as you americans were having your disastrous "Polar bear expedition" in Russia.