The intellectual roots of fascism are deep, intertwined, and often paradoxical. Most of its rudimentary ideas were formed in the second half of the 19th Century and were a reaction against the Enlightenment (modernism). Almost all of the major thinkers of the late 19th Century made some contribution to fascist thinking, though there were a myriad of minor players. Some of the major ideas that contributed to this unholy brew were:  romanticism, Darwinian evolution, alienation, and existentialism. Even Marxism must be considered in the shaping of fascism since some of the prominent leaders, Mussolini, for example, were Marxists, though the movement in some respects, evolved into a reaction against Marxism. Both Marxism and fascism opposed classical liberalism, both were revolutionary, and advocates of absolute government. The Italian strain of fascism looked more to Machiavelli and the idealism of Hegel, while the Germanic version took more of a cue from Nietzsche and the German romanticists.

Alienation

There can be little doubt that fascism was in some way the result of the alienation that followed the industrial revolution of the 19th Century. Science, technology, political changes, the philosophy of the enlightenment, and economic realities, created a barrier to man’s unity with the natural world. The philosophy of the 19th Century described the cosmos as a giant machine. That man was machine-like, and part of the greater machine, led to the feeling of not only being isolated from his fellow man, but also from nature. Veith says:

Fascism is essentially a response to the alienation that has been a part of the spiritual landscape of the West since the Enlightenment.

Logic and rationalism, with their cold analyses and denial of basic human impulses, have seemed stifling, heightening the sense of alienation. If objective knowledge is alienating, subjective experience is liberating and healing. Authentic existence comes from unleashing the emotions, cultivating the subjective and irrational dimension of life. The attempts to resolve the dilemma of alienation, understandable as they are, would find concrete and political expression in fascism.[7]

Romanticism

Romanticism became the antidote to alienation. It asserted the value of the natural world in that it was seen not as a machine, but as a living organism. Nature must not be approached with reason and intellect, but by experience, emotions, and irrationalism, as opposed to rationalism. The immanence of God (or gods) was emphasized while transcendence was denied. Romanticism was also characterized by nostalgia for the past and an admiration for the primitive. It was believed that primitive cultures were morally superior because they were more in tune with the natural world. Isaiah Berlin in his book: The Roots of Romanticism, says:

Fascism too is an inheritor of romanticism, not because it is irrational—plenty of movements have been that– nor because of a belief in elites—plenty of movements have held that belief. The reason why Fascism owes something to romanticism is, again, because of the notion of the unpredictable will either of man or of a group, which forges forward in some fashion that is impossible to organise, impossible to predict, impossible to rationalise. That is the whole heart of Fascism.[8]

Darwinism

In a sense Darwinism was a refutation of romanticism which held that by observing the beauty and harmony of nature one could be taught lessons of harmony and peace. Darwinism, however, saw the other side of nature, i.e., struggle, violence and cruelty. The law of nature is the survival of the fittest. Progress comes from ruthless competition, the strong destroying the weak. Darwinists theorized that if progress comes from struggle on the natural level, it must also come about that way on the social level (Social Darwinism). Darwinism gave new notions of heredity, race and environment.  The Nazis took Darwinism to its natural conclusion: If you can breed better sheep by selective breeding why not human beings (eugenics)? Zeev Sternhell says Darwinism:

stripped the human personality of its sacramental dignity. It made no distinction between the physical life and the social life, and conceived of the human condition in terms of an unceasing struggle, whose natural outcome was the survival of the fittest.[9]

Much to the chagrin of modern-day evolutionists, Richard Weikart has written a whole treatise on the Nazi application of Darwinism to race and culture. If you want the proof, you will find it in:  From Darwin to Hitler. In this incredible volume, Weikart quotes a German zoologist to show the extent of the Darwinian application among Nazi-fascists. The scientist was Robby Kossman.