Yet their ideologies were rooted in specific philosophical ideas – ideas which had many respectable adherents in their day.
One person who understands this is Jonah Goldberg, author of the 2007 book Liberal Fascism: The Secret History of the American Left from Mussolini to the Politics of Meaning. Ten years on, the book still holds up. Goldberg argues, provocatively, that fascism shared roots in common with what we call modern liberalism or progressivism.
People often argue over whether Hitler and Mussolini were "right wing" or "left wing." More to the point is that both men's ideologies had roots in the Progressive movement of the turn of the 20th century.
The Progressive movement was closely tied to the philosophy of Pragmatism: the belief that thought is a tool for action and change. In contrast to the ancient and medieval philosophers, for whom philosophy was the contemplation of reality, the Progressives were animated by the desire to mold reality and to harness knowledge for social betterment. Many in the vanguard of progressive thought initially were enamored of Mussolini and even Hitler, considering their dictatorships a useful "social experiment."