Well, despite Italy's victory in the war, it failed to achieve control over the Dalmatian coast and Fiume (despite a pro-Italian uprising in the latter). This led to a feeling of being sold out among the victors. Enter Mussolini. Mussolini's National Fascist Party built a base of support by violently attacking socialists, who had been involved fairly low level(albeit occasionally swelling) violence. In 1922, The National Fascist Party occupied the Po Plain and marched on Rome, forming a fascist government after the King feared Civil War. In 1923, the Acerbo Law was passed, which allowed the NFP to gain electoral victory disproportionately. In 1928, the NFP's control over Italy was completely solidified, and Italy became a one party state. Thus ends the story.

Italy did not come out of the First World War in good shape. Between 1919 and 1920 frustrated (and unemployed) demobilized soldiers aligned themselves alternatively with militant communist groups or militant fascist groups, both of which were bent on shaking up the social order. The socialist-dominated governing coalitions (headed by Nitti between 1919-1920 and Giolitti between 1920-1921) proved unable to deal with the economic and social crisis, and by the end of 1920 army regiments in Central Italy were in open revolt. Communist paramilitary activity was so widespread the years 1919 and 1920 in Italy are called the "Red Biennium."

The answer linked above is by no means comprensive, there certainly is room for more contributions.