in the standard game there is no restriction on japan fighting the USSR
in real life there would have been no phisical barrier to it either, breaking a treaty during war time carries no penalty, you arent going to get sanctioned by the UN or anything….because you probably already are.
in simulations however, account should be taken of the fact that
1. Neither power wanted to fight eachother becuase they just did in the 1930’s and it was a bloodbath
2. Conflict in that area is expensive because of distance and climate, and there is little reward in minerals or oil (that is, little that is easy to extract)
3. And both powers had larger concerns in other areas.
I am disapointed the game does not have a pact rule, i hope 18 infantry and vast distance can simulate it enough