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.

p. 180 - The Passing of an Illusion, The Idea of Communism in the Twentieth Century (1999)