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"Contrary to a 'horseshoe' theory, the evidence reveals increasing antisemitism moving from left to right."
(64.media.tumblr.com)
Hersh, Eitan; Royden, Laura (25 June 2022). "Antisemitic Attitudes Across the Ideological Spectrum" Political Research Quarterly.
doi:10.1177/10659129221111081
Horseshoe theory is dumb, but it's really just an observation of the loudest ideologies on the far left and far right, which both happen to be authoritarian. Authoritarianism becomes necessary as you move toward the extremes because you have to coerce some people/classes to accept the system. And it's true that real-world instances of both Fascism and Communism have been authoritarian, and so they share some things in common. It isn't a particularly nuanced or deep understanding, but it is true that authoritarian forms of gov't are authoritarian. The difference lies in the details. Communists used authoritarianism against capitalists and the nobility, and fascists used it against minorities. Horseshoe theory conflates "authoritarianism" with extreme Left and Right-wing ideologies. This contrasts against anarchism (and by extension the broad anti fascist movement), of course, which is extremely anti-authoritarian (hence why horseshoe theory completely falls apart here).
Why is this only necessary at the extremes? I don't want to accept the current system I live under, but I'm coerced into complying with it through force (police).
I'd argue that: 1) what is extreme changes over time, 2) a system of government being extreme de facto means it will have less support; the more support it has, the less extreme it is by definition, 3) the less support a system of government hass, the more force will be required to maintain it.
I am also under a system of government that is oppressive and monopolizes violence, but if the government had less popular support, I fully believe it would proportionally ramp up the oppression and violence. In fact, I'd argue that it's currently happening in the US.
I don't know that I agree with your definition of extreme. On the one hand, there's popularity of various ideas, and on the other, there's how much the idea differs from the way things are currently done. It's possible for an idea drastically different from the status quo to be popular, but it would still be considered extreme because of how big of a change it would be.