Established in 1912 by a confederation of "houses of labour", the USI led a series of general strikes throughout its early years, culminating with the Red Week insurrection against the Italian entry into World War I.

Syndicalism first arose in Italy at the turn of the 20th century, after the houses of labour (Italian: Camere del Lavoro) were established throughout the country.

[10] In its first year of existence, the USI organised numerous general strikes, including by workers employed in marble production, metalworking, construction, agriculture, rail transport and sailing.

[12] The revolutionary interventionists, led by USI general secretary Alceste De Ambris, favoured participation in the war as a means to prepare the country for a social revolution.

[13] But they were opposed by the majority of the USI's membership, which deposed De Ambris, expelled the interventionists and elected Armando Borghi [it] as the new general secretary.

[20] The USI was briefly able to continue its activities, organising a general strike in Milan in March 1921, but before long it came under attack by the rising fascist movement, which broke up its trade unions and harassed its members.

[21] The USI resolved to take direct action against the fascists; it formed armed anti-fascist detachments known as the Arditi del Popolo and reinforced its labour centres.

By April 1924, the union's legal activity was paralysed; it consequently reorganised itself into an underground organisation and led a number of wildcat strikes by miners and marble producers in Tuscany.