Why didn't Japan help Germany by invading Soviet during World War 2?

Part I

Modified from an earlier answer

A good part of the often dysfunctional German-Japanese alliance (I went into the other European Axis powers here ) was the great geographic distances between the two powers that made it hard to coordinate grand strategies. Geographic distance, however, was only one reason for the failure of the Axis coalition. Both Japan and Germany possessed very different strategic rationales behind their decision-making processes and these rationales only barely overlapped each other.

The Japanese non-invasion of the Soviet Union had its origin in both the labyrinthine decision-making process in the Japanese state and the immediate contingent situation facing the Japanese in late 1941, namely the American economic embargo on strategic war materials.

The grand strategic vision of both the IJA and IJN reached a major fork in the aftermath of the Russo-Japanese War. Without a common enemy (Russia) or a shared conceptualization of strategic space (Korea and Liaodong), the two services saw the future sources of Japanese power and security in different terms. The IJA favored a continental approach with a Japanese-led regeneration of the Chinese and the creation of various imperial satellite states within the East Asian mainland. The navy favored what became to be known as the "Southern Drive" for the natural resources and the seizure of the resource-rich lands of Southeast Asia. Both of these grand strategies had currency in the services prior to the Russo-Japanese War, but the interwar period saw this strategic thinking come into full flower. The IJN began to elaborate its southern strategy by designating the US as its main hypothetical enemy, while the IJA in conjunction with elements of the colonial apparatus in Liaodong created a puppet state Manchuria in 1931.

With regards to the Soviet Union, the IJA saw it as the inevitable great power threat to its ambitions in China. As such, the IJA devoted large resources to create a large field army in Manchuria, the Kwantung Army. Anticommunism became one of the chief hallmarks of the IJA officer corps and a number of officers longed for an invasion of the USSR. The IJN looked askance at these developments in China as a threat to its own domestic powerbase and as a diversion from any Southern drive. Inside Tokyo politics of the 1930s, the respective service ministries were able to take advantage of Japan's constitutional division of powers wherein the resignation of either the Navy or Army Minister could bring down a government. This created a political stalemate of sorts between these differing strategic visions.

The outbreak of war in China after the Marco Polo Bridge Incident did not really alter the strategic priorities for either service. The IJN used hostilities to seize Chinese territory like Hainan Island which could be used in a future Southern drive. The abandonment of the London Treaty system led to a new round of IJN construction, including warships designed to beat American quantitative superiority. The IJA for its part still kept up the establishment of the Kwantung Army and fought several sharp border clashes with Soviet forces. The IJA General Staff drew up two plans for a 1942 invasion of Siberia in 1939 at the behest of the Chief of the General Staff, Prince Ka'nin.

For their part, the Germans actually encouraged the Japanese to venture into the Pacific. Prior to Barbarossa, the German military planners in OKW had increasingly saw Japan as a vital counterweight against both the British and the US. A Japanese invasion into the Pacific would both tie up American naval power, thus preventing it from intervening in the Atlantic and by occupying SE Asia, Japan would be eliminating British access to the raw materials necessary for waging a modern war. Getting both Britain and America entangled in the Pacific would secure Germany's strategic rear while it took on the task of destroying the USSR. This was in keeping with the Third Reich's strategic thinking with regards to the Anglo-American powers in that it was in German interests to keep them preoccupied outside of areas controlled by Germany. OKW's 14 December report claimed the prognosis for the following year good for these four reasons:

I) Within the period left to it before the full mobilization of the American war machine, Germany would reach its military objectives in the east, in the Mediterranean, and in the Atlantic.

II) Germany would succeed, by political means, not only in inducing its allies to intensify their war efforts, but also in securing the periphery by bringing the flanking powers-hitherto neutral-of Turkey, Spain, Portugal, and Sweden into the continental defensive bloc.

III) The Japanese offensive would have enough endurance and momentum to tie down a substantial part of the Anglo-American potential in the Pacific for a considerable time.

IV) Under these circumstances the United States would not be able to conduct an offensive two-ocean war in the foreseeable future.

It was only well after Barbarossa had bogged down that German military attaches sought to encourage a Japanese invasion of Siberia, but by then the Japanese decision-making process had precluded such an action.

The stalemate about Japan's strategic direction broke in the summer of 1941. In an about face, the IJA gave a tacit support for the Southern Drive and the occupation of French-Indochina. The reasons for this volte-face were several. The German's swift occupation of Western Europe made these areas a highly tempting target. The bogged down nature of the war in China also made the IJA cast about for strategic alternatives that would deal the Nationalist regime a death blow. The bloody nose the Kwantung Army had received at Nomonhan in 1939 had underscored to the IJA that it needed a large-scale investment in heavy industry to modernize the IJA artillery and tank forces. The scale of the Soviet response at Khalkin Gol did throw cold water onto much of the IJA's planning, but the shift away from a Northern Drive was not universal within the upper echelons of the IJA. Alvin Coox's monumental study of Nomonhan concludes that IJA opinion on the matter was far from united, but debates between officers that believed Japan should wait and build up its strength through a Southern Advance or those that felt fortune favored the bold actually intensified. These debates came to a head during the summer of 1941. The IJA General Staff's Operations Bureau chief, Major General Tanaka Shinichi was a highly vocal proponent of the Northern Drive even after the non-aggression treaty. At one point in June 1941, Tanaka nearly got into a fistfight with the General Staff's planning chief Colonel Arisue Yadoru over a decision to strengthen the Kwantung Army. In these debates Tojo acted as a bellwether, alternating between positions and adding to the confusion. The new commander of the Kwantung Army, General Umezu Yoshijiro, began a process of trying to rein in the independence of this field army in the aftermath of Nomonhan. Umezu accomplished this by a policy of "keeping cool," and often tried to placate the hawks by promising that 1942 would be the ideal time for Japan to take revenge upon the Soviets. These debates all factored into the IJA's strategic reorientation by mutually reinforcing each other. But what likely animated the IJA most in this strategic reorientation, and often reinforced these other factors, was economic pressures.

When the war in China commenced, the Japanese government created an entity known as the Planning Board to marshal Japanese natural resources and create an put Japan on an autarkic war footing.The problem with autarky, as their German counterparts discovered as well, was that is much easier said than achieved, especially for a nation with such limited resources as Japan. In 1939, the Planning Board took an unprecedented step and had actually cut resource quotas to both services. Although protests from both the IJA and IJN got these cuts restored, the lesson to both services was clear: Japan needed raw materials. The IJA's plans for the Siberian invasion called for a massive amount of resources and the Planning Board's 1939 decision forced the IJA to adapt the more modest of the two plans as the basis for its future planning. The advent of war in Europe added further pressures on the Japanese economy as it cut off Japan from European imports. This left Japan's only major source of foreign supply the United States. This drained Japan's foreign currency and gold reserves at an alarming rate. The Roosevelt administration's denial of export licenses and other embargoes in 1941 signaled to the Japanese military leadership that America was using an economic weapon to control the direction of Japan's security. The US made it relatively clear that the restrictions would be lifted when Japan modified its behavior. Both the IJN and IJA already were hostile to letting the other control the direction of Japanese security policy for the better part of twenty-five years, and now were certainly not about to let a foreign power do so!