Fairly simple.
Somewhat different Soviet leadership, which
a) doesn't do the Great Purge (maybe just a Major Purge)
b) gets aggressive about regaining territories lost in 1918-1921 (and behind that, about spreading the Revolution)
and
c) Is clever enough to see that one can get away with aggression if one creates or takes advantage of ambiguous circumstances. For instance, disorder in the target country which seems to justify intervention to "restore order". A target government which is notoriously abusive. Occupation in the guise of establishing bases under a treaty of mutual defense. A territorial claim which has some historical basis and diplomatic cover. A territorial basis which can be justified by ethnic character. Inviting other countries to grab shares for themselves. Tricks like this worked for Hitler - for a while. Then he overreached and provoked war.
There were places where the Soviets could have used similar tricks. Plus the Soviets had influence through Communists in the West. And the USSR's frontiers were further away. If the USSR invaded say Estonia, what could any other power do about it? Oh, and don't annex the country, install a puppet regime.
So the Soviets embark on this program before Germany, which for whatever reason holds back a few more years. Germany even makes a surprise deal with the USSR to divvy up Poland (with the USSR doing the fighting). Since the USSR attacks first, it becomes the "aggressor state". The "Allies" fight, getting hammered by the massively equipped Red Army. The Soviets get cocky, and sneak-attack Germany (both to take out a potential foe, and to get at their enemies' homelands). But Germany hangs on, and becomes an "Allied country".
There you go.