What if the Japanese had attacked the Soviet Union instead of Pearl Harbour?
Japan tried testing the Red Army in both 1938 and 1939. The first time they lost, and the second time they got completely crushed. Japan had a military that was completely unsuited for fighting in Siberia, and after 1939 they knew it. It was this defeat (at Khalkin Gol if you want to google it), that convinced Japan to forget about ever attacking the Soviet Union, and started them planning for a fight with the US. And in late 1941, when the situation in the west was at its most dire, the Soviets did pull their best troops from Siberia to fight the Germans, but they never had fewer than 10 infantry divisions in Siberia, and when you look at the terrain, weather, the size of the operational area, and how lacking in mechanized forces the Japanese were, they clearly couldn’t have threatened Siberia enough to have given the slightest help to the Germans.
