Read up on the Paris Commune, read Homage to Catalonia by Orwell, read up on the anarchists from Manchuria. Those are just the bigger ones I can think of from the top of my head, but there are plenty more (usually smaller scale) examples. Also, read David Graeber's work, especially The Dawn of Everything like another user suggested.

The common point of failure for those, was being a smaller entity that was surrounded and attacked by imperialist forces; some of which received help from other, more powerful, imperialist forces that had a vested interest in these groups failing.

I'm trying to remain cordial and nice, but it's quite difficult when it seems like usually the people claiming "no viable alternatives have been shown to work" have never actually looked into any alternatives; it hardly feels like good faith argumentation.