Probably essential reading.
To begin at the beginning: Antifa—real name: Antifaschisitsche Aktion—was born during the street-fights of the 1932 Weimar Republic. It was founded by the Stalinist Communist Party of Germany (KPD), although various Communist “anti-fascist defense” units were associated with the KPD much earlier.
Anti-fascist Action’s sole purpose was to help the Stalinists combat other political parties for control of the streets in the revolutionary politics of the rapidly failing Weimar Republic.
And yes, they fought the Nazis.
But they also fought liberal parties, conservative parties, and anyone and everyone who got in their way. While these early antecedents were short-lived, it is useful to view Antifa in this context. More than anything, Antifa exists to serve as a tool of revolutionary politics in a failed (or failing) state.