Richard Wolin, a professor of history and comparative literature at City University of New York, has written an interesting book entitled The Seduction of Unreason: The Intellectual Romance with Fascism from Nietzsche to Postmodernism. Although its title is cumbersome (it seems authors these days must include explanatory subtitles, as it were, following colons, in every book title) the writing is not. It's a lengthy book, but unfortunately there's much to tell.
The title of the book is, shall we say, suggestive of its argument. If it had been written by an American or English philosopher, continental philosophers and their followers here would likely ignore it. But perhaps the author's status as a professor of comparative literature will lure them into reading at least summaries of the book, comparative literature buffs being far more capable philosophers than actual philosophers almost by definition as far as postmodernists are concerned.
It's not clear to me that the romance of intellectuals with autocracy began with Nietzsche. I think certain philosophers have always had a fondness for "enlightened" despotism. Even J.S. Mill in his more Romantic moments thought this would be beneficial, and under the influence of such as Coleridge dreamt up a kind of elite, a "clerisy", which would govern humanity wisely. This pernicious view has been with us since Plato, at least.
But fascism is a peculiar, modern form of autocracy, a hodgepodge of political, racial, quasi-religious, quasi-philosophical and nationalistic mumbo-jumbo, and as such grew out of what I think can fairly be called the "unreason" which resulted from the reaction to the Enlightenment which has been called (not very creatively) the Counter-Enlightenment. I personally prefer calling it the Un-Enlightenment. Some thought that because reason, in and of itself, didn't produce a paradise here on Earth, it's opposite should be unleashed upon the world.
It turns out reason's opposite could be all kinds of things when considered by European intellectuals. The emotions, the instincts, Jungian collective unconscious, the erotic, the spirit inherent in a particular race or purported race or nation. Whatever was unreasonable in human beings was glorified at least as much as reason, or what was thought reason, was glorified by other European intellectuals during the Enlightenment. This intellectual environment was ideal for the growth of fascism, a political/social/cultural philosophy dependent on the unreasonable, dismissive of moderation and tolerance, devoted to the irrational in humanity.
This led to what was extremely goofy in some cases, as Wolin points out. There is Georges Bataille, for example, a follower of Nietzsche who was fascinated with human sacrifice. He and others formed a little society which was devoted to the idea, if not the practice, of human sacrifice. It seems its members were willing to be sacrificed but not inclined to perform the sacrifice, sadly. Bataille was an influence on Foucault, who favored the Iranian revolution and the religious fascism of the Islamic clerics who took control after the Shah was overthrown, and others. I'm not sure what was going on in the case of Bataille and sacrifice, but suspect he thought this was the sort of thing unreasonable people would and should do, which is undeniable, and that there was something mystic and pagan involved in it.
Goofiness aside, though, the emphasis on unreason motivated the endorsement of Hitler's Nazism and Mussolini's fascism by a number of thinkers dissatisfied with liberal democratic politics and values. This is pretty well documented, and Wolin of course goes into some detail regarding famous Nazis and fellow travelers such as Heidegger and Paul de Man.
Mussolini's Italy and Hitler's Germany were once considered models of efficiency and, it's hard to believe, physical and mental health. It was thought that these nations had a vigor and intensity lacking in liberal democracies, for what may be considered spiritual reasons. They were one, a people united, not chaotic in the manner of the democracies and especially the Weimer Republic. Most importantly it seems to their intellectual devotees, they had abandoned liberal values as well; freedom, individuality, the conflict of ideas and worst of all, materialism. Money and business have always been the subjects of contempt for the learned as well as the aristocratic. Of course, the learned were also inclined to associate them with Jews, themselves yet another subject of contempt.
It's quite possible to overestimate reason, and to impute too much importance to it. It's also quite possible to underestimate the significance of the irrational in our lives. Man cannot live by reason alone, and it may be said that the Enlightenment thinkers erred in their zealous emphasis and reliance on it to the exclusion of all else. I think sometimes that those of the Enlightenment were drunk on their sudden freedom from the dominance of the Church.
But I find it impossible to blame the Enlightenment for the ills of our society, as it seems members of the Un-Enlightenment are inclined to do. It isn't reason or science which motivated the Holocaust, for example. Essential to that horror were concepts of the Volk and German Romanticism and mysticism. They were also essential to German expansionism and sense of mastery and special purpose; to rule the world and thereby save it. Heidegger warned against technology but eagerly joined in the glorification of Hitler and in according him the status of a demigod.
We're at out best when we think, and as Dewey said we think when we're presented with problems. Thinking involves problem solving and is essential to it. We don't think when we rely on mysticism, our so-called inherent nature, our "being", our vigor, our unity, religion, our race in making decisions. It's when we abandon reason as a means to solve problems that we produce monsters and follow them. We become followers only, in fact, as we don't think but rather feel; followers only follow, some better than others. Reason provides the best chance of understanding and solving the problems we face in life.
Unreason doesn't require justification. There's no process by which those seduced by it test its results, no questions are asked as unreason isn't subject to question or for that matter definition. Questioning, defining, testing have no place when reason isn't employed, but is instead shunned. The result is certainty, but at a terrible price, as the certain are thoughtless, intolerant and cruel.