What made Italian Nationalists upset after the World War 1 peace treaties were concluded?

Answer 1

Italian nationalists, including the early Fascists, were upset because Italy received less territory than they expected.

Like Germany, Italy spent much of the 1800s as a bunch of small countries struggling to form into a single large country. Italy (and Germany) finally unified in 1871; however, there were still lands around the Adriatic Sea that some Italians considered "unredeemed"- mainly in what's now Slovenia, Bosnia, and Croatia. Italian nationalists wanted to bring this Italia irredentiainto Italy as well, but Austria-Hungary controlled it at the time.

In the run up to World War I, Italy allied with Austria-Hungary (and Germany) because they were nervous about going to war with France because both Italy and France were arguing over some colonies in Africa. However Italians still generally disliked Austria-Hungary because Austria-Hungary had opposed Italian unification, and still controlled the irredentia lands. So when World War I finally started, Italy refused to join the war on Austria-Hungary's side, claiming that the war began because Austria-Hungary declared war first (which, technically, they did), and the alliance was only a defensive one.

Almost a year into the war, Britain convinced Italy to join the Allies. Britain promised that Italy would be rewarded by gaining Italia irredentia if and when they won the war. However, when the war ended, Italy only really got some territory around Venice, as well as "South Tirol", an area of the southeastern Alps. Most of the irredentia lands were instead given over to a new country called Yugoslavia.

Italian nationalists and Fascists were furious. They called it a "mutilated victory", and felt betrayed by the Allied Powers. Mussolini and the Fascists used this discontent to fuel their rise to power in the early 1920s.