Hitler may well have hoped that, by declaring war on the US, he might prompt the Japanese into attacking the USSR in Siberia, but if that was his intention he ignorantly miscalculated. The Japanese had already fought their war with the USSR and lost badly, something Hitler should have known, probably did know but disregarded.
The Japanese military in the 1930s was divided between those who believed Japan’s best strategy for material acquisition was to attack the sparsely populated Soviet far east–Siberia was a treasure trove of raw materials. The “Strike North” faction was made up primarily of Army officers. A large number of naval officers looked toward southeast Asia as Japan’s source of raw materials. But that part of the Pacific was guarded by the Americans, British and French, and the latter were regarded as strong world powers in the 1930s. The Russians were seen as weaker, so initially the “Strike North” faction prevailed. In 1938 and 1939 the Japanese fought increasingly large border battles with the Red Army, culminating in the August 1939 battle at Nomonhan/Khalkin Gol.
The Red Army inflicted a devastatingly bloody defeat on the Japanese, and the “Strike North” faction was fatally discredited. The Japanese subsequently signed a non-aggression pact with the USSR, which in turn allowed the Russians to sign a similar pact with the Germans.
Fast-forward to 1941. During Operation Barbarossa the Germans arrogantly dismissed the notion of Japanese assistance, thinking they could shatter the Red Army by themselves. By December 1941 it was plain that they couldn’t.
By that time the Japanese strategists of the “Strike South” faction had seen France defeated and Great Britain crippled. The remaining obstacle was the US. They committed themselves to a two-pronged strategy which involved knocking out the US Navy and grabbing as much southeast Asian real estate as possible, as quickly as possible. They failed to strike a mortal blow at the US Navy at Pearl Harbor and miscalculated the level of American outrage at that attack but simultaneously orchestrated a brilliant and rapid campaign of conquest in southeast Asia and the south Pacific. That effort pretty much tied their hands, and a second strike against the Russians was not considered a realistic option.
Richard Sorge reported the Japanese strategic decision to the Kremlin; freeing Stalin to pull troops from Siberia to counter-attack the Germans before Moscow. Hitler’s possible hopes to the contrary, by December 1941 a Japanese attack on Siberia was beyond rational consideration by Tokyo. But by that time Hitler himself was becoming increasingly irrational….
