The answer would be communism. Though Hitler is frequently understood as the face of fascism, he was really the front-runner of the Nazi Party. It is Italy’s Benito Mussolini who established the fascism philosophy. Mussolini required to reproduce the Great Roman Empire by usage of a totalitarian law and nourishing of the fear of communism.
Hope this helps!
Fascism was however a result of many fears, some of them were the fear of certain minorities taking over the main part of economies (in Germany people believed that Jews were too powerful), and the fear of poverty, as people were thinking that only a strong government can provide for its people.
But in many places communism was used as a direct demagogic threat to gain support.