The Spanish Civil War erupts into WWII. I would say that the capitalist side would consist of the US, Britain, France, Italy, Japan, and allies, and the communist side would consist of the Soviet Union, Germany, Spain, China, and allies. Unlike in our timeline, the British cannot afford to hide their support for Franco, and so they go full on supporting the fascists. The Soviet Union led by Trotsky does not sabotage the revolution, and the republican government is overthrown and a workers republic is created. I can imagine the war spreading to Italy and France, and it becoming a world war between socialist revolution and fascist counter revolution. With superior forces and the strong will of the people, I can imagine the communists winning this war. Mussolini's rule in Italy would end in a similar way to how it did in our timeline, with the communist countries assisting the resistance. Eventually the revolution also spreads to France and Britain, and with Germany's blitzkrieg tactics defeating the French like they did in our timeline, and the naval blockade and workers revolution in Britain, all of Europe is turned communist.