@FranceNeedsMorePower In general, you cannot attack a land territory that has neutral units. Either declare war against all countries in that territory, or you cannot attack.
If the tournament has some “house rule”, that changes this, I cannot say. It already has a “house rule” to allow Russians to enter China, so perhaps there is another to allow Japanese attacks against neutral Russians, which presumably would be ignored during fight.
Allowing the attack would break many paradigms of A&A, so I would not expect there to be such a house rule.
BTW, I’m using “house rule” = additional rules beyond G40.