Have Germany go communist after World War I, and ensure that the area between Germany and Russia stays under communist control.
The revolution could spread quickly enough to the old Austro-Hungarian Empire.
Britain and France will start to get worried, at this stage. I think they're the toughest nut to crack. Say they go fascist. Wehave a WW2 in the 30s/40s. By which time, there are communist rebellions in India, China, and Africa.
France and Italy fall fast, and Britain decides to focus on home defence. This turns out to be a bit of a mistake. Rebellions get bad across the Empire.
At the time, American labour struggles were pretty bad. America opts for isolationism for a the first year of the war, and by the time that's over, the world is becoming quite small. It finds itself dealing mostly with rebellions in South America.
The war finally comes to an end in the 40s with the nuking of London, Birmingham, and Manchester. Britain, America, South Africa, Australia, New Zealand and the strongest South American states enter into a protracted cold war, with proxy wars fought across the third world. But they're in a bad way. South Africa falls in the 70s over apartheid, and Britain struggles with a large labour movement.
The world is, by now, about 50% communist, 20% capitalist, and 30% neutral. Although, communist can mean various things. Some states, particularly the Scandinavian Social Democracies, encourage private enterprise. On the other hand, Britain has a large Communist Party which ocassionally forms coalition governments with Labour and goes on a nationalisation spree. China has a growing class of families that rule the economy. No state is a pure exemplar of any particular ideology or theory. Because this is the real world,and that rarely happens.