The formation of the modern Italian state began in 1861 with the unification of most of the peninsula under the House of Savoy (Piedmont-Sardinia) into the Kingdom of Italy. Italy incorporated Venetia and the former Papal States (including Rome) by 1871 following the Franco-Prussian War (1870-71). Prior to Italian unification (also known as the Risorgimento), the United States had diplomatic relations with the main entities of the Italian peninsula: the Kingdom of Sardinia, the Kingdom of the Two Sicilies, and the Papal States.
With the exception of the World War II years when Benito Mussolini’s government declared war upon the United States (1941-43), the United States has had warm relations with the Kingdom of Italy and, after 1946, its successor, the Republic of Italy. Currently, the United States and Italy share strong bilateral relations. Italy is a member of NATO and is a founding member of the European Union. [more]
source: U.S. Italy Relations – U.S. State Department, Office of the Historian