Were it only that simple.
The respective military services were actually each divided into two lines of command, for the Army, the War (Army) Ministry, which was within the cabinet, but directly – although nominally – reported to the Emperor, and the Chief-of-Staff which also directly reported to the Emperor in parallel to the ministry. Note that it was completely outside of any civilian oversight. The Navy was similarly set up. There were not only inter-service conflicts but intra-service ones as well.
While it seems mind boggling to non-Japanese, this is actually quite normal within Japanese organizations. I can only tell you of the frustrations encounter in the 25 years of living there.
The army did not all belong to the Strike North faction, and in fact key members of the rival Strike South were also army officers. A higher percentage of navy officers were in the South camp.
The division was not characterized by diametric opposition to each other. They agreed on many issues while violently (as in action of assassination for the leadership of their rivals) opposed on others.
The Strike North group was reactionary to the Taisho democratic movement and was centered on the Chosen group, officers from one of the former clans responsible for overthrowing the bakufu (shogun) government in the 19th century. They wanted to return Japan to her nostalgic past and key members were the ones who corrupted bushido to fit their ambitions.
The Strike South group was a reaction to the reactionaries. They were more worldly and knew that they couldn’t offload the zaibatsu industrial and merchant giants and other more ideological desires of the North group. Key members were from Satsuma, another of the key players in the Meiji Restoration. The IJN was build around the Satsuma group. Yamamoto was from that area as well.
Political maneuvering resulted in Hirohito’s 1920 engagement to a daughter of a royal family member from Satsuma. Chosen group members attempted to get it cancelling and to replace her with someone from their area, but that failed. The whole question of the degree of responsibility of Hirohito is beyond the scoop of this post, so we’ll leave it at this.
The North group was fiercely anti-Communist and that was a factor in their chosen desire.
The influence of Strike North group was already in decline prior to the fiasco against the Red army. Being on the wrong side of attempted coups does that for you. Hitler’s non-aggression pack with Stalin, which he didn’t inform Tokyo until afterward, was another factor.
All of this was made irrelevant on August 1, 1941 when the US established the oil embargo following on the heels of freezing Japanese assets. Once that happened, given the people in charge in Tokyo and Washington, war was inevitable.
One problem with alt-his is that they have to rewrite this part of history to what is equivalent to having Hitler buddy up with non-Russian Serbs or to not attack the USSR. At that point, it’s no longer WWII, either for Germany or Japan. The chain of decision which lead up to the war have to depend on certain factors.
Japan would never have invaded Siberia for Siberia. It was only to protect her gains in Manchuria and China.