Unlike in sea battles, you cannot attack a land territory containing units of a power with which you are not at war, even if there are other powers’ units in that territory with which you are at war.
This usually only comes up with Italy/Germany because Italy can declare war on Russia and take a Russian territory, then Germany (who has not declared war on Russia) can non-combat move units into an Italian territory (because they are allies and because Germany is at war with the UK)
The problem you are describing should not be able to happen. When Russia is not at war on the Pacific map they may not move units into territories of other countries (because Russia is technically not “allied” with them).
From Page 21 of the rulebook, under Phase 4: Noncombat Move, sub section: Where Units Can Move:
"If your power isn’t at war, you can’t move your units into territories belonging to another friendly power or a friendly neutral.
That being said, there is literally no reason why Russia should not declare war on Japan on round 1 and vice versa. There is no “binding” rule-legal agreement on whether or not Japan and Russia can declare war on each other unless you create one specifically.
There is one specific situation where this can occur. If Russia has units on the European side of the map (and is at war with either Germany or Italy), I guess your Persia is a good example, then Japan somehow shuttles over there or drives through India, then once they reach past West India, a single Russian unit prevents them from attacking or moving through to the next territory (on the European side of the map).
In the European manual on the page 15 blue sidebar you will notice a section called “Powers That Begin the Game Neutral”, subsection “Combat:”
“A power can’t attack a territory controlled by or containing units belonging to a power with which is it not at war…”
Note there is a special exception for sea zones but not land territories.