Look under Japan and Russia on page 33. It says the Jap and Soviet players are supposed to work out the details of any nonagression pact with the Soviet Union. So it appears it is actually up to the players to make the rules on this front.
I kind of assumed the italicized text was flavour text and not actual rules. If we are to treat these sections as actual rules then take a look at the italicized text in the United States section.
It reads “…only with the outrage felt by its people by a sudden and deliberate attack by an Axis power will the United States end its neutrality and go to war.”
Hmmm, so according to the rules if the Axis don’t make any sudden and deliberate attacks after turn 2 the US cannot enter the war. And how exactly do you decide whether an attack was sudden and deliberate?
Even though the rulebook says for the Japanese and Russian players to work out the details of the non-aggression pact it then goes on to provide rules for how and when Japan can attack Russia (as was mentioned there are no such rules provided for the Soviet player). Do the “rules” invented by the players overide the printed rules in the book. Example: Japan and Russia decide that they cannot attack one another for 3 turns. Does this now become a rule and overide the section below that states Japan can attack Russia on any turn?
Also, what happens if the Japanese and Russian players cannot agree to the terms of the non-agression pact? Do they start the game at war?
Under the Soviet Union, it only addresses its ability to declare war on European Axis and says nothing about Japan (like you noted). It doesn’t say they can’t declare war on Japan before Japan declares war on the Soviet Union, however.
Rules for boardgames generally need to be permissive in nature and not restrictive. That is the rulebook tells you what you are allowed to do and not what you are not allowed to do. For example: the rulebook doesn’t say that the UK cannot declare war on the US. You have to assume its illegal because it is not explicitly stated in the rules.
Once again, the rules tell you what you are allowed to do. And no where in the rules does it tell you how or when the Soviet Union may declare war on Japan.
This entire issue is a trainwreck and has a major impact on the flow of the game. I assume most people will game it that the Soviets can declare war on Japan at the beggining of any of their combat move phases (just like Japan can do to Russia).