A Brief History of Antifa
Mostly out of memory, but much of it can be looked up on the internet.
The popular belief that "Antifascism" is just opposition to political systems, like Italian fascism or German nazism, and that any person opposing them is an "antifascist", is naive, because the most important groups calling themselves "antifascist", especially those using the short term "Antifa", have a much more specific meaning.
The term originates from the 1920s and 30s, when the communist movement on one side, and groups mainly rooted in disbanded WW1 armies on the other, sought political power beyond traditional elites and with violence, leading to fierce rivalry and street battles.
For Germany, the communists attempted to gain a political monopoly, sending goon squads against any competitors, even smearing Social Democrats as "social fascists". When the Nazi party suddenly grew after 1930, many called the communists to unite with all other forces, standing against the Nazis. These would be alliances led by communists. The original version of the "Antifaschistische Aktion" logo had two red flags, symbolizing communists and social democrats united.
At the same time, communist leaders, namely Georgi Dimitrov, explained fascism as the armed force of capitalism and finance capital, bound to suppress the uprising communist movement. Thus, fascism had it's roots in the capitalist system, and only a communist, as one who wants to abolish capitalism, can be a true antifascist. This definition of "antifascism" is, with some modifications, still valid among Antifa groups: they often seek alliances with other opponents of neo-nazi groups, but their ultimate goal remains the erection of a communist regime.
Late 1960s: The "New Left" in Western Europe
In the late 1960s, major far-left tendencies emerged in many western universities, sometimes referred to as the "68 Movement". As opposed to traditional "working class" communist parties, these mainly had an academic background, and were not mere drones, bound to orders from Moscow. Marxism-Leninism, the dominant ideology in the USSR and it's Eastern European satellites, basically forbade own thinking and reduced supportes to dumb followers, something not acceptable for an academic followership. A hodgepodge of different communist groups came up, sometimes referring to Lenin, Trotzky, Mao or even Stalin. Often far more fanatic than traditional, east-bound communists, and mostly advocating violence to pursue their goals. By the time, many still believed in guerilla warfare and finally a revolution, to grab power. This was also the base of far-left terrorists, like Red Army Faction, Brigades Rosses, Action Directe or Weather Underground. Later, the small, militant groups would often be referred to as "K-Groups". For German far-leftists, viewing then West Germany as a still fascist country, or one that needed to be suppressed because of lingering fascism, became a core attitude. This was also the time when far-leftists started to court social and ethnic minorities, such as gays, feminists, environmentalists and the growing population of foreign "guest workers". Many "old 68ers" form the leftist elites in Germany and other countries today, having toned down or changed fanatic attitudes, to become socially acceptable.
1980s: House squatters and Black Blocs
To be safe from law enforcement, militant far-leftists would create spaces that were no-go areas to police. Tool of choice was squatting empty houses or entire streets, turning them into fortresses and bases for any kind of violent action. Fighting off police from occupied houses, as well as violent protests against nuclear plants led to the development of the Black Bloc tactics. These included terror and vandalism against uninvolved people and their property, but mostly restrained from killings or potentially deadly terrorist acts. Since most European countries have rather tight gun laws, and demand people to solely rely on police and legal system for self-defense, Black Blocs were, and still are, hard to counter.
The 1980s also saw the appearance of small, violent neo-Nazi groups, who had no more roots in original Nazism, but tried to restart it (or sometimes just troll with it) on their own. Later, the (biggest part of) Skinhead subculture joined them with thuggery. These soon became a major threat to everyone's public safety, a pretext for far-left street violence labelled as "antifascist", and to gain social acceptance for it.
With neo-Nazis and skinheads becoming an actual threat in the streets, resistence and support for Antifa groups grew. These would not just work as a vigilante group against skinhead thugs, but attack everything they considered "roots of fascism". Which could be just moderate right-wing attitudes, and ultimately, everything opposing a communist takeover.
Anarchism became an official part of Antifa ideology, represented by inclusion of the black flag into the "Antifaschistische Aktion" logo, frequent use of black-red flags, and the Soviet red star half black. There's an unresolved contradiction between absence of rule and ruling the streets with violence. Nearest suggestion is that anarchy here only means absence of law enforcement, enabling them for unrestricted violence.
Since then, the black-red icons have become symbols of violent street thuggery and vandalism, often fueled more by criminal lust, rather than actual goals to achieve by violence.
Around 1990: Collapse of Eastern Bloc, German Reunification, Hate Crime wave
The collapse of Eastern communism, and a German reunification becoming possible, was the sole, major defeat for the western communists, although many were no longer tied to Eastern governments. A huge part of the academic far-leftists became known as the "Anti-Germans", fundamentally equating all of Germany to Nazi atrocities and repeating fiercest wartime propaganda (no idea how they cope with being German themselves - probably they think they can get rid of it that way).
The early 1990s saw a temporary rise in neo-Nazi activity, and a soaring number of hate crimes, some of which were deadly. Militant communists saw this as their lifeboat and future base for violent action. Since guerilla warfare and revolution had to be considered impossible, an underground empire was the method of choice to exercise power. Most "revolutionary" and violence-advocating communist groups, including the terrorist Red Army Faction, jumped on the Antifa bandwagon, and only appeared under this label since then.
Since this time, an almost all-encompassing network of Antifa groups exists, supported by parts of media and political establishment, with violent acts being prosecuted only half-heartedly (police and judges are often attacked by media when seriously prosecuting them).
While far-left groups had little support in the 1990s, and many basically perceived themselves as enemies and oppressors of the average population, the trend towards "harsh" capitalism, with lowering wages, dismantling of social security and welfare cuts led to a "left renaissance" in the mid-2000s.
2016: Across the Pond. Trump, Alt-Right, Nazi Internet trolls
The political turnover of 2016, namely the perceived rise of the Alt-Right, increasing activity of internet trolls using Nazi imagery and racist memes, with no German sources and audience, brought a perceived, and possibly real, fascist threat to the USA. While smaller incidents with European-style black blocs and "antifascist" groups had occurred before, many leftists perceived Trump as an actual fascist takeover, requiring even violent resistance. The label "antifascist" granted existing groups support in the mainstream, often turning a blind eye towards the questionable background. In some cases (read further into article!), journalists even directly supported criminal black bloc vandalism that randomly targeted peoples' property.
US political tradition of free speech greatly differred from the authoritarian approach towards extremist ideologies in Europe. Suddenly, these traditions are at stake, and infringements become feasible for some, including enforcement by a completely extralegal militia. One rhetorical trait of European, especially German, Antifa agitation is to proclaim a permanent state of emergency, due to an impending fascist takeover, making abolishment of civil rights and an unrestricted permission for violence by them "necessary". While American Antifa has some differences, this is their least common denominator. It might be a very significant impact on political culture in the USA, as a backlash to Trump. It can bring back an extremism that had been mostly eliminated in the "red scares" long ago (not supporting some of the methods applied then).
Are they equivalent to neo-Nazis?
This is hard to judge, because both commit various violent and criminal acts while underground movements, and likely become mass murderous when taking full power. Ideologies are different, and far-leftists tend to come sugar-coated, as opposed to the obvious evil of fascists or nazis. In the (unlikely) case that a Nazi-like regime takes over in the US, perhaps a 100 million people would be immediately on the hit lists. A communist regime has less clear definitions of enemies, so anybody might become target. Maybe, as once in the USSR, just because secret police has to fulfill a kill quota and randomly grabs people from the street.
Perhaps Antifa is the lesser evil compared to actual neo-Nazis, but it's better to do everything to not get reduced to a choice between these.
What is definitely appropriate, is to equate the black blocs, perhaps not to Communist regimes or German Nazism, but to those American organizations, who sought to maintain invisible empires from the right, through underground armies in disguise. Only that these disguised in white instead of black, and burned crosses instead of cars.