Antifaschistische Aktion, commonly known under its abbreviation Antifa, was a militant "anti-fascist" organisation in Weimar Republic started by members of the Communist Party of Germany (KPD). Under the leadership of the committed Stalinist Ernst Thälmann, the KPD viewed fascism primarily as the final stage of capitalism rather than as a specific movement or group and therefore applied the term to all other parties.
The then-center Socialist Democrat Party (SPD) and affiliated groups had established the "Iron Front" that sought to defend liberal democracy and the constitution of the Weimar Republic. Antifa was formed partly as a counter-move to the SPD's establishment of the Iron Front, which the KPD regarded as a "social fascist terror organisation".
Modern US Antifa uses the two flag logo and the hammer & sickle, among other symbols that should give mainstream American pause.
Link: https://en.wikipedia.org/wiki/Antifaschistische_Aktion