Bolshevism and Fascism were heresies of socialism. - Richard Pipes

No prominent European socialist before World War I resembled Lenin more closely than Benito Mussolini. Like Lenin, he headed the antirevisionist wing of the country’s Socialist Party; like him, he believed that the worker was not by nature a revolutionary and had to be prodded to radical action by an intellectual elite.

[T]he view that anti-Communism equals Fascism remained obligatory in countries subject to Communist censorship until the advent of Mikhail Gorbachev’s glasnost. It was prevalent also in foreign ‘progressive’ circles. Western scholars who had the temerity to link Mussolini or Hitler with Communism in any way or to depict their regimes as genuine mass movements risked verbal or other forms of harassment.

For historians of the left even to raise the question of affinities between Soviet Communism and ‘Fascism’ is tantamount to conceding the possibility of a causal relationship. Since ‘Fascism’ for them is by definition the antithesis of socialism and Communism, no such affinities can be admitted and the sources of ‘Fascism’ must be sought exclusively in conservative ideas and capitalist practices. In the Soviet Union this trend went so far the under Lenin, Stalin and their immediate successor, it was forbidden to use the term “National Socialist.”