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The original left/right distinction dates back to the French Revolution. During a brief episode, supporters of the monarchy found themselves sitting on the right and opponents (republicans and anarchists) on the left.

Later, the term conservatism was applied to the ideas of Edmund Burke, who defended a kind of constitutional monarchy.

In that sense, the US founding fathers were radical left-wing revolutionaries. However, that happened a bit before the French Revolution, so the term is anachronistic.

WW1 broke the power of monarchies in much of Europe. Fascism rose in the interwar period between WW1 and WW2 as a kind of Ersatz-Monarchy. The fascist leader did not present himself as the true heir to the throne, but as a man of the people. He did not claim his position "by the grace of god", but might invoke "providence". As such, a stout monarchist might genuinely call fascism left-wing. They would also think the US Republicans are left-wing extremists; at least insofar as the Republicans are republican.

Since fascism is in many ways similar to the more authoritarian versions of monarchism, fascist leaders gained the support of old monarchists. There weren't many of those around, but they still controlled key positions in government and industry. You were not a high military office, government official, or plain rich, if you had not kowtowed to the king.

Because of all the overt similarities in ideology, demographic support, and policy, fascism has always been considered a right wing ideology by any normal observer.

They got you! Authoritarians LOVE rebranding and confusing language, calling themself democratic people republic. Or calling themself national socialists depsite murdering the disabled! It makes no sense and they love doing it, because it confuses people like you!