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Whenever the topic of fascism arises, whether it's a fascist group being prevented from meeting, or a gang of far-right thugs attacking someone, often someone will demur in the following way:
'You like to use the word fascism, what is fascism in your mind? That word is used to scaremonger.'
Fascism is fairly messy as ideologies and movements go, eluding being very concisely defined, but a sketch of it follows.
Fascism is an extreme nationalist ideology. It is based on some idea of 'rebirth', harking back to a mythical golden age where times were simpler, purer, and more honorable. It generally gains traction during a time of economic crisis or national disgrace, and that 'rebirth' is proposed as a way to regain national pride, racial pride, male pride, and so on.
Fascism is a far-right ideology but it often borrows tactical and economic ideas from the left, for instance syndicalism / direct action in the 1920s and often welfare state policies today. The original Nazis were called the National Socialist German Workers' Party (NSDAP) and used leftist rhetoric for a time to garner popular support from the downtrodden.
Fascism fully accepts, and in fact worships, hierarchy as part of the 'natural order', rather than seeking the classless society which we seek. Practically it involves collaboration across classes, with business owners supposedly uniting with workers in the national interest.
Fascism is highly hostile to personal liberty, asserting that the individual only matters insofar as they are part of the organic whole of the state. The state is the most important thing and every person should be an instrument to improving its strength, even if that means not having freedom of thought, expression, to unionise, bodily autonomy, and so on.
Usually fascists target at least one marginalised group as a scapegoat for society's problems. This is a way to channel people's legitimate frustrations to their own ends of gaining state power. It's also deeply convenient to the ruling elite who are let off the hook for actually causing these problems, and sections of that ruling class often end up supporting the fascists because of this. So fascists blame unemployment, lack of healthcare, alienation, desperation, fear, on migrants and people of other races, and straight cis men's sense of frustration on women and queer people having more rights. The latter is important as fascism is always a men's movement overwhelmingly created by men for men.
Fascists consider socialists of all stripes to be their sworn enemies, being ideological opposites. As people organised in class solidarity presents a serious challenge to their desire for political power, fascists always try to destroy left organisations, whether trade unions, campaign groups, or general political organisations like the WSM.
Fascism in many ways is the gun capitalism keeps under the table in times of systemic crisis, but it's important to remember that fascist movements aren't simply pawns of the current ruling elite and can come into conflict at times due to differing interests and the struggle for power.
In conclusion, we can differ on the finer points of what fascism is, but anyone with even a slight acquiantance with fascism who cherishes freedom, co-operation, truth, and creativity, will want to oppose fascist attempts to gain ground at every opportunity. We've played that one out before. The practical point is that fascists don't usually walk around saying 'as a fascist I believe we should open up the gas chambers again ...', they use tricks to conceal their true politics by trying to appear ordinary and reasonable. There's a skill to spotting these tricks, which takes a bit of practice, and learning that skill is, if we think about it, a matter of basic survival.