Military history of Italy during World War I

Although a member of the Triple Alliance, Italy did not join Germany and Austria-Hungary when the conflict started in 1914, on the ground that war was initiated by the two Central Powers while the Triple Alliance was a defensive bloc. Italians protested for the lack of consultation before Austria issued the ultimatum to Serbia and invoked a clause of the Triple Alliance, according to which both Italy and Austria-Hungary were interested in the Balkans and whoever changed the status quo in the region had to compensate the other; Austria-Hungary refused any compensation before the end of the war. In May 1915, after secret parallel negotiations with both sides, the Italians entered the war as one of the Allied Powers, hoping to acquire the Italian-speaking "irredent lands" of Trento and Trieste (in Italian discourse the conflict was described as the "fourth war of independence" against Austria) and other territories (German-speaking South Tyrol, the largely Slavic-speaking regions of Istria and Dalmatia where Italians lived in coastal cities, some colonial compensations) promised them by the allies in the 1915 treaty of London.

Italy opened a front against Austria-Hungary along the Eastern Alps and the Isonzo river. Fighting was marked by trench warfare and attrition. On the Julian sector, the Italian army launched numerous offensives and made several conquests (most significantly Gorizia in 1916 and Bainsizza in 1917), but both sides suffered heavy casualties. On the Asiago plateau, in 1916, an Austrian offensive was followed by an Italian counter-offensive. Italy was forced to retreat in 1917 by a German-Austrian offensive at the Battle of Caporetto, when the Central Powers were able to move reinforcements to the Italian Front from the Eastern Front.

Further offensives by the Central Powers were stopped by Italy at Monte Grappa in November 1917 and on the Piave River in June 1918. On 24 October 1918, the anniversary of Caporetto, Italy launched the battle of Vittorio Veneto: Italian troops achieved a breakthrough, recovered the territory previously lost, and moved into Trento and Trieste by the time fighting ended on November 4 (a date celebrated as National Unity Day in Italy). The centuries-old Habsburg Empire collapsed. Italian armed forces were also involved in the Western Front (taking part in the Second Battle of the Marne, around Bligny, and in the subsequent Hundred Days Offensive), the African theatre, the Balkan theatre and the Middle Eastern theatre.

At the end of World War I, in the Paris Peace Conference of 1919, Italy was recognized a permanent seat in the League of Nations' executive council and the possession of many promised territories: Trento and Trieste, whose annexation completed national unification, South Tyrol, Istria and some colonial compensations. Territories in Dalmatia (although also included in the 1915 treaty of London) and the town of Fiume (despite a sizeable Italian population) were not assigned to Italy as they were claimed by Yugoslavia; a part of Italian nationalists (the followers of Gabriele D'Annunzio) accused the Allies of "mutilating" the Italian victory and occupied Fiume without royal assent. The quarrel between Italy and Yugoslavia would be solved by the treaty of Rapallo (1920), with Italy annexing the Dalmatian capital of Zara and Fiume becoming a free State. The post-war period saw increasing political violence, eventually leading to the establishment of Mussolini's Fascist government in 1922.