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.
Damn, I'll go tell Ho and Giap the Japanese weren't fascist
Did Japan win? As far as I remember, they lost WWII and got kicked the fuck out of the rest of Asia. And they were acting in those countries as an imperial power. Repelling a foreign empire is completely different from electing homegrown fascists. Which is my point. Nowhere did fascists gaining power advance a revolution in the same country (or at least if it has happened it's very rare for it work out that way; like I said, I don't really know much about Nicaragua's situation).