I will find out, I managed to get the Soviets activated about six game weeks ago in April 43... From what I have seen so far, if properly fortified and manned, the Japanese border bases can hold their own on a 2 attacking divs :1 defending div ratio, but the Soviets seem to have much more units to commit than the Japanese can muster for defense, esp. when having triggered the Soviet activation by pulling out too many units from Manchuria...Furthermore, I expect Soviet flanking movements and the risk of the border bases being cut off. I don't have units to spare to guard the flanks, so timely retreat will be the only option. So I guess long-term prospects are bleak. My fighters (Jacks and Tonies) with 70+ exp do fine against the Soviet Airforce, but the sheer numbers of Soviet planes is scary and my fighter units need to rotate often. Furthermore, my opponent has moved US heavy bombers to Soviet airfields and started bombarding the Home Islands... Next time I insist on a house rule against that. I have already notified my opponent that the Soviet activation means 'game over', but we will continue anyway since it doesn't make much sense to start a new game with AE pending. So if there is anything new in my game, I'll post it.
EDIT: I should add that I play CHS, not stock.
EDIT 2: castor troy - are you kidding?
I´m not kidding, why should I?
Perhaps the situation in your game is different because you seem to have activated the Russians by accident and furthermore, I don´t know anything about the situation in Manchuko in CHS. Though I can say, in stock, BigB and in Nikmod I´m pretty sure the Russians can be defeated, with the chances for the Japanese beeing 70:30. Seen it often enough in stock already. Of course every or at least most times this was done with reinforcements from the Chinese theater.
In my game vs Swift we will soon see a Japanese attack on Russia also (Nikmod) as soon as the winter is over as all those Russian bases seem to be in the cold zone, making it harder for the attacker. I have crushed China (no biggy in Nikmod, even though Nik has strenghten the Chinese compared to stock) and now the troops are redeploying for the assault on Russia. I still have thousands of assault points in China so no, not all Chinese divisions will attack Russia but certainly enough to make me very confident that I will knock out the Russians too. I expect Russia to be defeated at the end of 1943.