Depends on your POD. If you're speaking about an early one, perhaps you could have more of the attempted revolutions of the late 1910s and early 20s to be successful, eg Finland going red and being incorporated into the USSR after the Civil War, also Hungary, Slovakia and Bulgaria. One of my planned scenarios in my Tunguska event timeline was where London has a socialist revolution, with the royals going into Canada, and than providing more help to the Bolsheviks, both militarily and financially, allowing a stronger, more established and more efficient Far Left movement. If you're talking WW2/Cold War, maybe D-Day either fails, is delayed or is slower progress than OTL allowing the Soviets to occupy more of or all of Germany and Austria, and maybe even 'liberate' Denmark and Italy from German oppression. Or maybe America is more laid back in it's foreign policy allowing more communist forces in South East Asia and AFrica.