Taierzhuang 1938 - Stalingrad 1942: Insight into a Blind Spot of WW2

Questions crucial to the development and outcome of WW2 are a blind spot in the conventional wisdom on, and histories of, WW2:

Why did Japan invade China and what difference did it make to WW2?

Japan intended to invade Siberia (USSR) at the time of Operation Barbarossa but attacked Pearl Harbor instead - what made Japan change its target?

What saved Russia (USSR) from having to fight - and being defeated in - a 2-front war against Germany and Japan at the same time in WW2?

Ja…