Japan didn’t attack the Soviet Union during Germany’s attack as well is because of its own problems and the overwhelming force the Soviet Union has.

By June of 1941 the tension between Japan and the West had reached a breaking point. Japan a year earlier had taken over the now Vichy controlled French Indochina. This drove the US to cut off imports of metals like copper, steel, and iron to Japan. Japan heavily relied on these imports and now were without out them. Things only got worse as Japan tried to trade with the Dutch colonies and British colonies in the South but were also turned away. The situation by August of 1941, reached a turning point and now all oil exports to Japan were banned. Japan had almost no oil herself and relied on the US for about 80% of it’s oil. And the Dutch East Indies and British Malaya were also not trading with Japan. This meant that Japan would soon run out of oil, estimate put an 18 month time period before Japan ran out of fuel. Japan had the option of leaving China, Manchuria, and French Indochina to get these imports back, but they refused. They knew if they did this the Chinese would strike back at them one day. And all that hard won ground would be for nothing.

Japan made the choice then to take the Dutch East Indies and the rest of the South Pacific by force. The only problem was the US and her fleet. Japan knew it could not attack the South without dealing with the US first and so it planned it’s attack on the Pacific fleet. This brings me to my second point, Japan could not deal with this and an attack on Russia. You can’t compare the alliance Japan and Germany had with the Allies alliance. While Germany and Japan were friendly with each other, they could not help each other or support each other as easily as the Allies could. Japan could not receive any aid from Germany nor could Germany send any with it’s meager fleet. Japan also faced down the Soviets in the border conflict in Mongolia in 1939. Japan was utterly humiliated in the battles of Lake Khasan and Khalkin Gol as the Russians had better mechanization and tactics during the battles. Japan’s army was not prepared to fight a fully mechanized army. It was well suited for the environment in Southeast Asia and prepared to deal with Chinese, colonial garrisons and other Asian soldiers. Many Japanese armies lacked heavy guns and heavier tanks, though it makes sense as they had to ship everything to and from the mainland. Every ton counts so putting a 50 ton tank on a ship means 50 tons of artillery, ammo, soldiers, and other equipment would not be shipped. Wasting ships and oil that are always in high demand. The US also had to deal with this problem but had a better industrial base and infrastructure in place to help this problem.

Japan would later sign a neutrality pact with Russia in 1941. Japan did not want to fight Russia after the border conflict and knew it was too weak to fight them. They instead put faith in their navy to defend Japan, and their eyes laid southward to the European colonies. Japan would always keep an attack on Russia an option, however they had to be in complete chaos for this to happen. And not even at the height of Barbarossa and Typhoon did the Soviets panic and capitulate. Germany however hoped Japan would attack at one point during the Barbarossa campaign. Germany thought that if Japan could pin down the Soviets in Siberia, they could end the war then. Japan had no interest in helping Germany and only watched the situation to see if Russia would collapse. Japan had its own problems to deal with as it planned to take Southern Pacific and attack the US. Even if it wanted to, Japan could not plan 3 simultaneous operations at the same time on top of it’s war in China. They barely had enough supplies as is, this would stretch the Japanese too thin. However after the attack on Pearl Harbor, Hitler thought he could bring the Japanese into the war.

Hitler, contrary to popular belief was not upset about the Japanese attacking Pearl Harbor. Hitler was actually quite surprised and impressed the Japanese even attempted an attack of this scale. Seeing as Barbarossa stalled out and his troops had not taken Moscow, he wanted to bring the Japanese into the war. So a few days after the attack he declared war on the US. To this point, Germany and Hitler had been cautious to attack US shipping as not to antagonize the US into the war. Hitler respected the industrial might and resources the US would bring to bear in the war. But he believed he would have dealt with British and Russians before the US even declared war. Now after Pearl Harbor, he knew it was only a matter of time before the US joined the conflict in Europe and so he declared war on the US. This was to try and unite the Japanese and Germans against a common enemy. Hitler wanted the Japanese to coordinate an attack on Russia while their fleet pinned down the US for a time until Germany could bring Russia down and take it’s resources. This was overestimating the Japanese capabilities at the time, but it was what Hitler thought the Japanese were capable of. Japan however had no plans of invading Russia as it was securing the South, soon it would have all the resources it needed alongside fighting the US Navy.

Japan did not and could not attack the Russians. Even if they wanted to, the Russians would push them back after they pushed the Germans back. Then the Japanese would face a resurgent Red Army it had no hopes of stopping. Japan had a multitude of problems and an invasion of Russia would not solve any and only add some more.

Source:

Toland, J. (2003). The rising sun: the decline and fall of the Japanese empire, 1936-1945. New York: Modern Library.

Internal politics. The invasion of the Soviet Union, called Hokushin Ron ("Northern Road"), was actually a very popular idea advocated by many army officers, including the establishment from 1931-36. Advocates of this plan were resisted by the Navy, which advocated Nanshin Ron ("Southern Road", or war with the Western powers), and the Toseiha ("Control Faction") of the army which advocated no war at all. By 1936, advocates of Hokushin Ron lost much influence due to a failed coup by some of their supporters, and by 1940, the US had become an existential threat to Japan through imposing an oil embargo. This left all factions with no choice but to side with the navy.

The 1931-41 period represented the formative phase for Japanese strategy in World War 2. That year, junior officer Kanji Ishiwara staged an incident which led to the invasion of Manchuria without authorization from Tokyo or the civilian cabinet. This act of "loyal insubordination" greatly empowered the faction of the army that supported him, which were overwhlemingly the conservative establishment of the Japanese army. In December 1931, the most important member of this group, General Araki Sadao, was appointed War Minister by Prime Minister Inukai. The next year, Araki supported the assassination of Inukai. At this point, Araki wanted the army to control the cabinet, but was stopped by Prince Saionji, who forced a compromise by installing an admiral as Prime Minister. Nevertheless, Araki remained as War Minister until his retirement for health issues in 1934, at which point he was granted a peerage and remained the force's ideologue.

In September of 1932, just months after the assassination of Prime Minister Inukai, Araki outlined plans for the Japanese army and coined the term "Kodoha", or Imperial Way Faction. Believing in the power of Yamato Damashii, or the "Japanese fighting spirit", Araki advocated Seishin Kyoikyu (Spiritual Training), which consisted of lectures on samurai texts and frequent beatings to build this spirit. Operationally, the doctrine of his faction resembled the traditional principles of the Japanese army - infiltration, surprise, rapid maneuver, encirclement, a light force, and the bayonet. Kodoha is often falsely labeled as a "radical" faction - in practice, it was supported by the old guard of the Japanese army and new graduates of the Imperial Army Academy (because the academy had been teaching these lessons for decades), and represesnted the "establishment" of the IJA.

Kodoha advocated the invasion of the Soviet Union due to belief in victory, which was not unjustified. War gaming and simulations were universal throughout the Japanese army and navy at this point, and nearly all conducted predicted victory against the Soviets in the East for geographic reasons. Siberia was sparsely populated, had a poor road network, and consisted of forests - ideal terrain for Japanese infiltration tactics, and poor ground for the mechanized Soviet force. Further, there was a highly defensible chain of mountains not far from the Manchurian border, straddling Lake Baikal, which itself was a long and impenetrable geographic barrier. Most importantly, the main Soviet supply route - the Trans-Siberian Railway - was close to the Manchurian border and could be cut at any point. Ironically, Kodoha, which was demonized in Japanese and Western sources alike following its defeat in the factional battles of 1936, was proven correct in some of thier assumptions by the Soviet-Japanese border war of 1940.

Kodoha was not without opponents. A clique of officers who represented the most promising Imperial Army Academy graduates of the pre-WW1 era formed an opposition called Toseiha (Control Faction). This opposition, led by Generals Tetsuzan Nagata and Tojo Hideki, had no name for itself and was created simply to oppose Araki - Toseiha was a derogatory name created by Kodoha. Because they were top graduates, Toseiha's members had been sent to observe the Western Front of World War 1, and saw the destructive power of massed artillery and tanks. They believed the army had to establish a dictatorship, forcibly industrialize Japan, and modernize the army on Western lines.

Until 1936, Kodoha was unquestionably ascendant. However, in February of that year, officers aligned with the faction staged a failed coup, which resulted in a purge of the Kodoha brass. At this point, the main dispute in Japanese war plans were between Toseiha, the Navy, and one clever Daimyo named Konoe Fumimaro.

The highest ranking nobleman outside the Imperial family, Konoe became Prime Minister in 1937, appointing Araki Sadao as Minister of Education, where he engineered a minor comeback for Kodoha by restructuring military education along the lines of their principles. The appointment of Araki was carefully calcluated to divide the army, making Konoe the most powerful civilian Prime Minister since the 1920s. Konoe proceeded to launch a full scale invasion of China in defiance of both the army and the navy. The plan was ideologically motivated - Konoe embodied a strand of proto-wokeness which gained steam in Japan throughout the interwar. As early as 1918, he wrote an essay condemning the Versailles Treaty as an "Anglo-American Centered Peace", in which he claimed the principles of self-determination and democracy as espoused were hypocritical because they seemingly applied only to white people. Konoe's racial consciousness accelerated in 1934, when he visited his son who was studying at Princeton, and observed rising anti-Japanese sentiment in the United States.

Rejecting the contention of Japan's elder statesmen that civilized conduct and good diplomacy would result in the West seeing Japan as an equal, Konoe believed Japan would never be accepted and had to build its empire without concern for Western opinion. Konoe's regime appealed both to Kodoha by resuming plans regarding Hokushin Ron (ironically taken over by Tsuji Masanobu, a Toseiha faction who played a major role in stopping the 1936 coup) and restructuring education around their ideas, and to Toseiha by restructing the economy. To do this, Konoe founded Kikachu-Cho, or the Cabinet Planning Board, and passed legislation giving it sweeping powers to reorder the Japanese economy on wartime footing.

This was not enough for the Konoe cabinet to survive indefinitely, however. Seeking the "eradication" of the Chiang Kai Shek government, Konoe dispatched the army on an endless series of battles, which, though victorious, had not come remotely close to forcing the Chinese to submit. In 1939, Konoe resigned as Prime Minister, but was recalled the very next year.