You might want to consult my post on "what is fascism?" to get an idea of the context of the ideology before reading this.

Fascism as a political movement originated in Italy as a result of the First World War. Many leading Fascists including Benito Mussolini were originally members of the Italian Socialist Party who were expelled or quit in 1914 because they supported Italy's war effort; unlike their German or French counterparts, the Italian socialists were vehemently opposed to supporting the conflict. The expelled members formed fasci which can be translated as "band" or "league". The leagues did not necessarily resemble the fascists that we're all familiar with; they still contained a considerable syndicalist or socialist presence and rejected the idea of scientific racism (Mussolini himself denounced the idea as absurd until it became a politically expedient tool for him during WWII). They were originally left-leaning nationalists (though the original fasci certainly contained the sort of reactionaries that would come to define fascism) with an obsession with militarism and irredentism, the idea that Italy deserved some slices of land on the Adriatic.

While the Fascist movement began in Italy, its intellectual foundations were the writings of two French thinkers: Gustave Le Bon and Georges Sorel. Le Bon's work, "Psychologie des foules", argued that men are qualitatively transformed when gathered into a mass; they lose their capacity to reason and instead of thinking independently, manifest hereditary racial attributes. The civilized man in a crowd descended For Le Bon, the hereditary trait that was manifested in the Latin crowd was its emotionality and evocativeness. Latins, unlike Anglo-Saxons, craved authoritarianism and a strong and militaristic leader; ‘le type du héros cher aux foules aura toujours la structure d'un César. Son panache les séduit, son autorité leur impose et son sabre leur fait peur.' Despite being written in the 19th century, Le Bon's book eerily reads like a "How to be a fascist dictator: for dummies!" handbook.

Unlike Le Bon, Sorel was a socialist revolutionary and was more influential on fascist practice than theory. He advocated the use of political violence by the working class for revolutionary ends which obviously appealed to the militarist fasci. He argued that violence was not abhorrent in and of itself; instead it was virtuous and conferred vitality when used in a revolutionary context. Like Le Bon, Sorel believed that people were fundamentally irrational and required a "political myth" to be successfully mobilized for political action like strikes or riots.

Based on these views and their virulent support for military intervention and their conception of virtuous violence, the fasci and Italian Marxists/socialists became more and more antagonistic and eventually regarded each other as political enemies. The aftermath of WWI led to an explosion in fascist leagues' membership as great numbers of alienated young men returning from the front lines were upset with civilian society and sought military hierarchy and masculine comradeship that the fascists provided. This led to a "rightward swing" in the fascist movement that was solidified during Italy's "Red Years"; a wave of factory occupations and peasant seizures of land in the aftermath of WWI. The increasingly organized (Mussolini founded the nucleus of the Italian fascist party in 1919), militant and right-wing fascists sided with landlords and bourgeois factory owners and violently repressed striking workers and peasants. Oftentimes, fascist gangs would roll into "red" villages in the countryside in trucks provided by the landlords and brutalize the population, often forcing them to drink castor oil.

The fascists were now a considerable movement with wealthy and powerful backers in industry and politics, who thought that they could use the fascists to rid Italy of socialism and Marxism which would lead to their March on Rome in 1922 which ended in Mussolini's appointment as Prime Minister.