As the soviet Union, you are always standing on the gaters of hell, caught between the Germans and the Japanese.  The best defense is a strong offense . . . a little bit of luck.  Never, never let the Germans hold Karelia for an entire turn.  Keep pumping out tanks and infantry and try not to lose your fighters.  Hold off the Japanese, which should be easy enough because by the time they reach Moscow, they are becoming over-stretched and can only make small attacks.  To slow them down and make life easier for yourself, move everything but one infantry to Yakut and prepare to make a counter-offensive as soon as they take SovietFar East (unless you are not restricted, in which case attack Manchuria right away).  Once your allies gear up and make their offensives, you should be free to start wreaking havoc among the fascist swine!