@Sovietishcat It looks like USSR is not at war - at least on the Pacific side of the map. That means it is still under the restrictions of being a neutral power - at least on the Pacific side of the map. So to be able to reinforce US controlled Korea USSR must leave its neutral status there by DOW on Japan. Reinforcing US controlled Korea would not affect Mongolia, as Russia would not attack Korea.
Rules:
Europe 1940.2, page 37
“As a result, if the Soviet Union is at war with Axis powers on only one map, it is still under the restrictions
of being a neutral power (see “Powers Not at War with One Another,” page 15) on the other map …”
Europe 1940.2, page 15:
“While a power remains neutral, it operates under even tighter restrictions. A neutral power
can’t move land or air units into or through neutral territories. It can’t move units into or through
territories or onto ships belonging to another power or use another power’s naval bases, nor can another
power move land or air units into or through its territories or onto its ships or use its naval bases.”
Europe 1940.2 page 40:
“If the Soviet Union has attacked Korea or any Japanese-controlled territory bordering these Mongolian territories while Mongolia was still a strict neutral, Mongolia will remain neutral and not ally itself with the Soviet Union.”
(‘attacked’ bolded by me)