USSR is virtually impenetrable if it employs a stack defence strategy in Belorussia. On its first turn, only counterattack Germany's opening assaults where overwhelming odds are possible and no strong German tank retaliation can occur (this means most likely in Scandinavia). All your infantry except for a token force in Karelia, Leningrad and Ukraine should rendezvous in Belorussia. Add all the tanks and artillery that can reach, and move up an AA gun for added defence. This should give you about thirty units; too many for even the German tank stack to assault. Build new units in Leningrad or Stalingrad depending on whether the German concetrates tanks north or south. Wait for them to come adjacent to Belorussia and counterattack them; hold your fire when faced only with infantry. Eventually the Germans will become frustrated enough to assault Belorussia directly. If you've been supporting it each turn with your new units, there's almost no question you'll survive the assault. Germany will have to rebuild, but you can rebuild even faster and the odds will only continue to tilt in your favour while you buy more time for your allies.