I'm listening to an episode of Opening Arguments in which the host digresses to mention that Soviet Russia didn't declare war on Imperial Japan when it joined the Allies against the Axis. Stalin was vulnerable in the east and didn't want to deal with Japanese soldiers while fending off Hitler's invasion on the Osfront.
Why, though, didn't Imperial Japan declare war on the USSR in support of their Nazi allies? Hitler declared war on the USA in support of Imperial Japan and both wars were initiated by Axis surprise attacks.
Was there any political fallout when Japan failed to follow through on the alliance commitments Hitler was faithful to? (that's awkward sentence to write)
This is a really good question because in addition to the Japan's membership in the Axis alliance, Japan and the Soviet Union had a very contentious relationship in the years preceding the Second World War.
In summation, despite decades on contentious relations with the Soviet Union, Imperial Japan did not declare war on the Soviet Union due to its defeat against the Red Army at Khalkhin Gol in 1939, Hitler’s temporary peace with the Soviet Union from 1939-1941, and the more alluring targets in Southeast Asia.
Part of the contention between the two states stemmed from their history. Japan's ascendance to the world stage came at the expense of the Russians during the Russo-Japanese War of 1904-1905. The Imperial Russian Navy suffered two wholesale defeats at the hands of the recently industrialized Japan. Additionally, the Japanese army effectively blocked of portions of Russia’s east coast by taking over the railways in that region. Contemporary racism made this defeat even worse as the white European Russians lost to the Asiatic Japanese. The Russo-Japanese War served as one of the main reasons behind the 1905 Revolution in Russia, and this revolution and its aftermath became one of the key steps in the Tsars eventual abdication in 1917.
Even though the Japanese victory in this conflict served as one of the early steps to the downfall of the Imperial Russian government, relations between the new Soviet Union and Japan remained contentious throughout the 1920s and 1930s. Much of this came as the result of Japan’s designs on Soviet held Siberia. After Japan invaded Chinese controlled Manchuria, the threat to Siberia became even more pressing. Additionally, Japan and Poland had very good relations, so harmonious that in the early 1930s the Soviet government became convinced that the two nations had signed a secret agreement to conquer the Soviet Union. While there was never a secret agreement between Warsaw and Tokyo, the Japanese rebuffed Soviet attempts to agree to a non-aggression pact during this same period (early 1930s). In addition to border clashes between the Soviet Union and Japan in 1935 and 1936, relations soured further in 1936, when Japan joined the Anti-Comintern Pact, an agreement between fascist governments to block the spread of Communism.
The Soviet Union and Japan did fight a series of border clashes from 1935 – 1939. While neither side formally declared war on the other, these small scale border incidents culminated in large battles by the end of the decade. The final conflict, the Nomonhan Incident/Battle of Khalkhin Gol, involved almost 90,000 troops (57,000 Soviets and 30,000 Japanese) resulted in a non-aggression pact between the Soviets and Japanese. Despite an initial success, the Japanese were defeated due to deficient tactics and inferior tanks. After coming to this conclusion, the Japanese government sought a formal peace with the Soviet Union.
Peace between the Soviet Union and Japan was further solidified with the signing of the 1939 Molotov-Ribbentrop Pact between Nazi Germany and the Soviet Union. Stalin’s main foreign policy goal of the 1930s was to prevent a two front attack by Germany and/or Poland in the West and Japan in the East. Germany’s reasons for seeking this agreement is a completely different conversation, so for the purpose of this topic, when Nazi Germany began inquiring the Soviet Union about securing a lasting peace, Stalin found his opportunity to avoid a two front war. Japan was caught completely off guard by this seemingly sudden change in German foreign policy. Most significantly for Japan, the Molotov-Ribbentrop Pact meant that if Japan were to fight the Soviet Union, it would be doing so alone. After its defeat at the Battle of Khalkhin Gol, the Japanese government did not think that an independent struggle against the Soviet Union was feasible. It therefore did not prepare for a future invasion of the Soviet Union, as Nazi Germany did despite a formal peace.Therefore, when Nazi Germany invaded the Soviet Union in June of 1941, the Japanese government was not in a position to support its allies.
Finally, an invasion of the Soviet Union was not worth the price. While Siberia did offer some of the resources Japan needed, the Battle of Khalkhin Gol had demonstrated to the Japanese government that this region would not be captured easily. Furthermore, South East Asia was a far more tempting target as it possessed the necessary resources and could be captured much more easily. Even though this meant going to war with France and the United Kingdom, both nations focused their efforts on defending their home territory against the Germans, meaning that Japan was not fighting these powers alone.
Again, in short, after learning first hand that the Soviet Union was more than a match for the Japanese military, Japan committed to its efforts to conquer Southeast Asia, rather than fight a potentially devastating war just to support its ally.
I pulled from The Library of Congress World War II Companion, edited by David M. Kennedy and Bloodlands: Europe between Hitler and Stalin by Timothy Snyder.
Much more can be written on this, but /u/kieslowskifan had a number of replies at and under "Why did Hitler and Mussolini declare war on the US to support Japan when Tojo/Emperor Hirohito didnt declare war on the USSR in support of Germany and Italy?" . This main reply covered Japan's war decisions; two replies below cover Japan's use of southern war materials, and the consequences of Khalkin Gol. As for "Hitler declared war on the USA in support of Imperial Japan", kieslowskifan addressed that notion in the top level reply (click on the title when looking at the above replies); since you didn't make that the focus of your question, I didn't search for more detailed replies elsewhere.
But this is not to discourage discussion. More questions, data, and debate are welcome.
I made a related question a few months ago which perhaps could help you: https://www.reddit.com/r/AskHistorians/comments/80fbbs/was_imperial_japan_afraid_of_the_soviet_union/
Now Im no historian, but I think I've read enough of it to draw some personal conclusions on the why events played like they did, and I think there are a few factors that can help to explain this:
The practical factor: Japan wasnt informed by Germany of its plans to invade the Soviet Union, they only learned of it a few days before the fact through unofficial communications with their ambassador in Berlinm they were informed on the 6th of June, the invasion began on the 21 of the same month, now Im no military historian, but going by common sense I suspect a few days wasnt nearly enough to organize an offensive against the Soviet Union (a nation they had already engaged and lost against in Khalkin Ghol), especially when they were engaged in other theaters.
The political factor: After the disastrous battle of Khalkin Ghol, the Japanese military heavily began favoring abandoning plans for Asian expansion and focusing in the Pacific front, now it should be noted that following the declaration of Operation Barbarossa the Japanese did began developing plans for the invasion of the Soviet Union: Kantoukuen, and began heavily fortifying the Kwantung army... this however never coalesced into a finished plan, and by the time they had built up the forces to perform such a plan the Pacific Expansion faction of Imperial Japan had already won the political battle, and while the Kwantung Army remained over a 1,000,000 strong, they found their resources more and more diverted towards other fronts, and as /u/Bangelo95 pointed out oil (and rubber) were of outmost importance for the Japanese at this point, it simply was perceived as far easier at the time to invade the Dutch Indies than Siberia I think.
The military factor: Now, this is the point were I start to make assumptions and guesses, but hear me out, previous to the Battle of Khalkin Ghol, the Japanese Army had a very contemptuos perception of the Soviet military steming from their victories during the Ruso-Japanese War and the Siberian Expedition, but the army the Japanese faced at Khalkin Ghol was very different, their tanks just werent designed to fight the Soviet models, their logistical lines were no match for the soviet ones, where a Soviet army was designed to stretch itself over 700 or so km without much problem, while during Khalkin Ghol there are stories of Japanese soldiers drinking motor liquid because they ran out of water, their doctrine even considering deployments further than 300 or so km as unwieldy, etc... the Japanese Army I think just wasnt a match gun-by-gun to the Soviet one at that moment, and the changing materiel requisitions of the Kwuantung Army following the battle with more vehicles and tanks imitating Soviet vehicle numbers, I think show that the Japanese Army were aware of this, in fact /u/nsorlov makes a very good case I think in this post:
All in all I've always been of the belief that the Japanese were "scared" after Khalkin Ghol of openly engaging a fully mobilized Soviet Union, thats a personal opinion and it might (probably) be wrong, but I think there were a lot of practical concerns that made such a thing impossible in the time frame in which it was available.