Date: 1920 – Annexation of Northern Italian Territories
Sometimes the realities of nationhood and shifting international borders affect citizenship. Italy's borders have changed since 1861, but there's one set of border changes that is particularly relevant in many citizenship recognition cases. Around 1920 Italy officially annexed certain northern territories that were part of Austria in the early 20th century. These areas include parts of Trentino, Alto Adige/Südtirol, Friuli-Venezia Giulia (particularly Trieste and Gorizia), and Belluno. Local residents of these areas legally became Italian citizens on this date.
According to Law no. 379 (14 December 2000), if your ancestor was born and resident in present Italian territory that was part of Austria in the 20th century, but your ancestor emigrated (i.e. left Austria and Italy) prior to July 16, 1920, you cannot be recognized as an Italian citizen on the basis of descent from that particular ancestor.
All other jure sanguinis Italians, regardless of historical fact (Rome and Venice actually joined after 1861, as examples) are legally considered Italian citizens from birth or from March 17, 1861, whichever is later.
Example: Mario was born in Alto Adige (also known as Südtirol) in 1898. Military records indicate that he was resident in Alto Adige in 1921, and other records indicate he emigrated to Canada in 1922. (His village in Alto Adige was part of Austria until 1920.) He never naturalized. Mario became an Italian citizen on July 16, 1920, and he retained his citizenship for his entire lifetime since he did not naturalize as a foreign citizen. Since there is documentation that he was a resident of his village (a part of Austria in the early 20th century) on or after July 16, 1920, his descendants can be recognized as Italian citizens if they otherwise qualify.
There were also some 20th century border changes involving transfer of Italian land to the former Yugoslavia, but those changes affect fewer citizenship recognition cases. Nonetheless you may have to do a bit more research if your ancestor was caught up in any such border changes, and the law is a bit complicated.
Italy's relatively brief experience maintaining an overseas colonial empire significantly enlarged the Italian diaspora. However, Italy's citizenship laws treat Italian descendants living in former Italian colonies the same as other Italian descendants.