Italian nationalism builds upon the idea that Italians are the ethnic, cultural, and linguistic successors of the ancient Romans who inhabited the Italian Peninsula for over a millennium. The origins of Italian nationalism have been traced to the Renaissance. Italian nationalism first arose as a potent political force in the 1830s in the Italian peninsula under the leadership of Giuseppe Mazzini. It served as a cause for Risorgimento in the 1860s to 1870s. Italian nationalism became strong again in World War I with Italian irredentist claims to territories held by Austria-Hungary, and during the era of Italian Fascism.