Japan's goal in Siberia was oil correct? Wouldn't capturing the oil in the Dutch East Indies have met this need? I know there are large mineral deposits in Mongolia that the Japanese wanted (and still want actually) and the Soviets wouldn't just let them have, but wouldn't the defeats in 39 and the inability of the army to change capabilities have caused them to come to some agreement over China with the Soviets? I think that the Soviets would rather have a China dominated by a friendly Japan rather than hostile Western powers or a Nationalists government.
Oil didn't really figure. During the Siberian Expedition, the Japanese had pushed as far as Buryatia, bringing with them over fifty thousand Japanese settlers. (A vaguely stated desire for living space was also part and parcel of this, along with a desire for large amounts of territory for its own sake.) The
zaibatsu, especially Mitsubishi, Suzuki, and Mitsui, set up subsidiaries run by and staffed by these settlers to control local mines, forests, and fisheries. Mineral wealth in particular caught Japanese interest. During the short four years of the expedition, they managed to extract enormous quantities of gold and platinum especially. The region's oil was mostly exploited in northern Sakhalin at that point, and the Japanese did make a point to seize those rigs in 1920 in retaliation for Bolshevik atrocities in Nikolayevsk, along with the island's coal mines.
The Japanese obsession with Siberian resources should therefore be seen in that vein. Oil wasn't really relevant, but it was at least there; mineral resources, timber, and living space were seen as more important.
Japan's opposition to the Soviets should also be viewed through an ideological lens of fanatical anti-Communism and a conviction that a
Soviet attack was inevitable and could only be met force-on-force, with the seizure of Siberia and Mongolia as buffer zones and imperial properties to follow.
As for the Soviets' part, in no way did it make sense to back a powerful, anti-Communist Japanese empire in China instead of the divided Guomindang. The Japanese were an imminent threat and were one of the few states seriously able to use the resources of a conquered China against the USSR; even the West, insofar as it could ever even theoretically dominate China - and by the 1920s and 1930s it really could not - would be preferable to the rule of a Japan that still played host to
Kodo and the
Kokuryukai.