Once you read "Homage to Catalonia" and "The Battle for Spain", you start to see patterns in leftist movements everywhere...it's always the Communists, the Anarchists, the Trotskyists, and the Republicans...
The Anarchists: The true revolutionaries. Idealistic utopians. Usually from disadvantaged backgrounds. Want to change everything about history, society, and culture. Usually lack organization and a coherent vision, but fight very hard. 
The Republicans: Center-left squishes. Want evolutionary leftism through democratic elections. Intent on building a big tent, so they ally with revolutionaries while trying to temper their fervor.
The Communists: Also called "Tankies", they seek the backing of external superpowers. Highly organized but with very ad-hoc ideology, they're focused on infighting and seizing power from other leftists. Their style and beliefs usually veer into "red fascism".
The Trotskyists: Doctrinaire leftists who resist both foreign control and big-tent moderation. They naturally end up fighting the Communists, who typically crush them.
Things usually seem to go the same way. The Anarchists' fervor eventually burns out, sometimes leaving lasting cultural or institutional change. The Republicans eventually fail to hold together their big tent. The Communists crush the Trots and try to take over...
If the Tankies take over and win the war you get the USSR. If they take over and lose the war you get Franco. If they fail to take over, things usually turn out much better.
How much positive change the Revolution produces seems to depend on A) how far the Anarchists can push stuff without burning out or sparking too strong of a Reaction, B) how long the Republicans can hold the big tent together, and C) whether the Tankies can be stopped.
Anyway the moral of the whole story seems to be: A) Revolutions are very hard, and B) Tankies are bad news.
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