well, since no one commented on this one...
The USSR is in a unique position of either being the poor cousin or sought after bride.
A key strategy for the USSR is playing the Allies and the Axis off eachother. Both sides no that Russia can tip victory in their balance. Playing on the assumption you are out for "World Communism" neither side is really your friend but can be useful
And always bully Japan, they will most likely agree to a non-aggression pact so you will leave them alone.