"To understand this relationship we may start with what has become an accepted observation: Stalinized Bolshevism and National Socialism constitute the two examples of twentieth-century totalitarian regimes. Not only were they comparable, but they form a political category of their own, which has become established since Hannah Arendt."
https://en.wikiquote.org/wiki/Fascism's_relationship_with_other_political_and_economic_ideologies