Antifa is just a nickname for antifascism. The history of today's antifa goes back as far as the 1920s. The movement arose in response to the rising power of European fascists and the far-right, authoritarian, ultranationalist regimes marked by suppression of opposition and forced regimentation. Anti-fascism arose in the US in the 1970s and 1980s in response to administrations seen to be leaning toward fascism.
Opponents of antifascism often refer to antifa as an organization, but that's giving us way too much credit. In reality, small, autonomous groups - crews - typically operate locally to stand against public functions of fascist and racist organizations that threaten violence. These little groups vary enormously in their ideology and strategies, since there is no such thing as a centralized antifa manifesto or procedure manual. Fascist and racist public functions are rarely organized in rural towns and small cities, so antifa groups are found mainly a big cities - if you can find them at all. A much larger number of supporters include capitalists like myself, anarcho-communists, socialists, Democrats, Libertarians, and people of no particular political stripe other than a commitment to stand against what we see as a highly destructive evil.
Other causes and groups are sometimes seen with antifa. When racist, religionist, and nationalist groups hold an event, counter-protesters can always be found. Some may be organizations such as NAACP chapters or BLM activists. They may also be individuals - moms, dads, students, retirees, political activists, and church groups - who don't explicitly identify as antifa. When antifa confronts racists at a public event, police are frequently and understandably present. Anarchists may be drawn to the event to protest the oppression of the police state. They may even be carrying a flag that resembles, but is distinct from, the antifa symbol. This may cause confusion among those not familiar with either icon; they may even conflate everybody present with some amorphous "black bloc."
What is antifa trying to accomplish? We must choose carefully to whom we accord the authority to speak for the movement. In my opinion, the most reliable and consistent is Antifa International. Here, in their own words, is who they are: "So we're anti-fascists because we a) oppose fascism and b) are willing to step up & do something about it. That's it, there's no other political ideology you have to subscribe to." Also, "Antifa is a big ol' non-sectarian tent where people holding all kinds of different political beliefs can come together in their mutual hatred of fascism & desire to rid the world of it."
In my dialogs with Antifa International, they reluctantly accepted that I identify as both a capitalist and as antifa. Capitalists have a very bad reputation in many quarters [Corporate Capital's War on America] and many of them wouldn't fit in very well. When I assured them that I'm no Carl Ichan, they were quite welcoming. They don't even mind that I wear the tee shirt.
Strategies and tactics are far more loosely defined and implemented. You can count on an antifa presence whenever an event is planned by a fascist organization. They have been known to confront fascists by shouting them down during their speeches, jeering at their parading, and a few have met violence with their fists. That's the headlines. Antifa International was asked how to promote antifa strategies. Their answer is found here. Besides disrupting fascist activity, the advice included collaboration and support, helping to build a local anti-racist youth culture, and reaching out to targeted communities such as migrants, LGBTQs, and minorities with offers of help and support. There's no place in the antifa playbook for violence against persons or property unless it's required in direct confrontation of fascist violence. An antifa-identified person who destroys property or harms innocent people in the name of any cause is acting outside the norms and values of antifa.
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