As I said above - Japan initiated Khalhin Gol while at war with China. Without war in China, it was very possible, that Japan would have inflated that into full-blown war. Thus, the start is in May 1939. Allies already guaranteed Polish independence and all. Allies want Soviet help against Germany - Wehrmacht is next door to them, while IJA is half the globe away. In fact, it is possible, that Allies sign on with USSR (since SU is even less demanding in that situation), and Berlin gets a 2-front war in 1939, against Paris and Moscow. Wehrmacht is divided, has MUCH less resources, gets beaten, and true horrors of WW2 never happen.
USMC had to invade islands and do head-on attacks, with no possibility of flanking the enemy. Manchuria offers enough space for flanking maneuvers.