Why does Marxism in practice always result in totalitarianism?
Well... centrally-planned, hierarchical forms of socialism certainly has. Unfortunately, no country has attempted to implement a horizontal, decentralized one. For example I'm talking about concepts like libertarian socialism, participatory economics, and others.
However, I'm aware of one microcosm with it's roots in 1960s era Marxism that has survived to this day where there is no central planning, and no authoritarian ruler, The Rainbow Family:
"The Rainbow example threatens governments. It shows that people can live without rulers, without yielding their voices to representatives. It demonstrates that people can be responsible for themselves and maintain peace without coercion or force, without police. It is a model of a true participatory democracy, 'Government by the People.' The European Gatherings are bringing people of different ethnic and national backgrounds together to discuss their common future; to dream of a world without armies or wars. The Rainbow Family is the antithesis of a police state. It challenges all entities that govern by fear instead of cooperation. For them, the Rainbow Family provides the 'threat of a good example,' one others might follow (Chomsky 1987)."This does not mean I'm advocating we all gather in the woods as utopian hippies. I'm just trying to point out that your stated cause and effect would take only one example to disprove (of course this is not the counter example as they aren't a country and the analogy begins to break down). What I'm trying to say is that Totalitarianism ≠ Socialism no matter how much you are trying to equate the two.
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Works Cited:
Niman, Michael I. “People of the Rainbow: a Nomadic Utopia.” People of the Rainbow: a Nomadic Utopia, University of Tennessee Press, Knoxville, 1997, p. 214.