[dude with glasses in a communist t-shirt, arguing] I'm the only leftist here, your opinions are TRASH
[dude holding a theory book on smug, arguing] Read theory you losers, you're all WRONG
[dude in an anarchist hoodie, arguing] Nuh-uh, I'm the only leftist here, you're SHITLIBS
[the three dudes are now caught in a cartoon fight, glasses gone flying, punches everywhere, while a firing squad of nazis are targeting them with rifles]
[a confused nazi asks] Why… why are they still arguing?
The Soviet Union was established before the Nazis existed. There was no need to ally with anarchists against them when they didn't exist yet and waged war.
A better example would be the Spanish civil war where communists, liberals and anarchists did fight on the same side until infighting broke out due to an ineffective and non-authorative government. Meaning they failed to establish a leading ideology which could have prevented this.