This assumes USSR1 has taken West Russia but did not attempt Ukraine.
I move all available infantry and tanks to Karelia (this includes 1i1t from WEU by TP). That should be 6 tanks 9 infantry.
1 infantry is left in Ukraine. All other available units across Europe are sent to Eastern Europe (7i1a2t). All the air lands there as well after hitting the Gibraltar BB and any exposed targets.
I use the Mediterranean TP usually to ferry troops from Libya to France so I have a total of 4i1a1t there.
Finally, I build 8 tanks in Germany.
Now I have three Army Groups:
Group 1: Karelia stack and EEU tanks. Can hit ARC or WRU.
Group 2: EEU infantry/art and GER tanks. Can hit BEL or UKR.
Group 3: EEU airforce. Can hit ARC, WRU, BEL or UKR.
The important thing is Russia cannot defend 4 territories against these two stacks. On G2 I can conquer the weaker territory with each of these stacks. Because the rear stack can pass through the territory held by the front stack, on G3 I can send 16 German tanks, 6 fighters, 1 bomber, and some infantry to hit either Caucasus or Russia on G3.
How would you counter this opening as USSR/Allies?