I know this is an impossible question, the future is chaos, but we can at least identify and discuss some key factors.
When I say "successful leftist revolution" I mean one that happens soon enough to prevent, or have non-negligible odds of preventing, the collapse of human civilization as we know it due to climate change.
In all likelihood I think we're fucked as far as climate change is concerned, but we should still try to improve our odds. There's no prize for correctly predicting that we're fucked.
Cuba and Russia, yes. Germany and Italy, no.
Cuba wasn't a fascist victory, it was decades (centuries, really) of imperial dominance. Russia also wasn't a fascist victory - the libs had just won and that paved way for the communists to carry the torch.
Batista was definitely one nasty motherfucker and so was the tsar. I don’t know of Batista being explicitly racist but the tsar blamed Jews for everything and the Black Hundreds were definitely a proto-fascist gang (like the Freikorps).
I’m reading Cuba Libre right now, it’s all about Cuba during the revolution, with many parallels to the current situation in the USA, although things have actually not gotten as bad here as they were in Cuba under Batista.