The USI was founded in 1912, after a group of workers, previously affiliated with the Confederazione Generale del Lavoro (CGL), met in Modena and declared themselves linked to the legacy of the First International, and later joined the anarcho-syndicalist International Workers Association.
The most left-wing camere del lavoro adhered in rapid succession to the USI, and it engaged in all major political battles for labor rights - without ever adopting the militarist attitudes present with other trade unions.