Italy entered into the First World War in 1915 with the aim of completing national unity: for this reason, the Italian intervention in the First World War is also considered the Fourth Italian War of Independence, in a historiographical perspective that identifies in the latter the conclusion of the unification of Italy, whose military actions began during the revolutions of 1848 with the First Italian War of Independence.

After the Capture of Rome (1870), almost the whole of Italy was united in a single state, the Kingdom of Italy. However, the so-called "irredent lands" were missing, that is, Italian-speaking, geographically or historically Italian lands that were not yet part of the unitary state. Among the irredent lands still belonging to Austria-Hungary were usually indicated as such: Julian March (with the city of Fiume), Trentino-Alto Adige and Dalmatia.

The Italian irredentism movement, which aimed at the reunification of the aforementioned with the motherland and therefore their consequent redemption, was active between the last decades of the 19th century and the early 20th century. It was precisely in the irredentist sphere that the theme of the need for a "Fourth Italian War of Independence" against Austria-Hungary began to develop in the last decades of the 19th century, when Italy was still firmly incorporated in the Triple Alliance; also the Italo-Turkish War was seen, in the irredentist context, as part of this theme.

When World War I broke out in August 1914, Italy declared neutrality [it]. Although nominally allied with the German Empire and the Empire of Austria-Hungary in the Triple Alliance, the Kingdom of Italy did not join the Central Powers; in fact, Germany and Austria–Hungary had taken the offensive while the Triple Alliance was supposed to be a defensive alliance. Moreover, the Triple Alliance recognized that both Italy and Austria-Hungary were interested in the Balkans and required both to consult each other before changing the status quo and to provide compensation for whatever advantage in that area: Austria-Hungary did consult Germany but not Italy before issuing the ultimatum to Serbia, and refused any compensation before the end of the war. Italy negotiated for a better deal with the Allies, especially in terms of gaining territory from the Austro-Hungarian Empire. Italy's neutrality also favored the Spanish's neutrality.

However, Russia had its own pro-Slavic interests in the Balkans; with her ally Serbia claiming much of the same territory sought by Italian irrendists, complicating negotiations. Russia's position was greatly weakened by heavy military losses throughout the first year of the war. In the face of insistence from London and Paris, Russia, by April 1915, abandoned its support for most of Serbia's claims and accepted terms for Italy's entry into the war, which would limit the Russian strategic presence in the postwar Adriatic. Italy accepted the Allies’ offer in which Italy would receive a slice of Austria and a slice of the Ottoman Empire after the defeat of the Central Powers. This was formalised by the Treaty of London. In 1915, Italy entered the war joining the Triple Entente (i.e. the Allies).

Public and elite opinion was split on the wisdom of entering the war, for either side. Italy was very poorly prepared, the army was not well trained, and there was too small an industrial and financial base. A handful of leaders made the basic decisions, notably Prime Minister Antonio Salandra and especially the two foreign ministers Antonio di San Giuliano and Sidney Sonnino. They optimistically expected that victory would bring new territories and new glory, hopefully closing some of Italy's internal conflicts. The country made a fundamental contribution to defeating the Central Powers and Italy was recognized as one of the "Big Three" top Allied powers. Under the Peace Treaties of Saint-Germain, Rapallo and Rome, Italy gained a permanent seat in the League of Nations's executive council. Despite the increased international prestige, expectations of territorial expansion fell short. After the war, Italy never received any portion or "slice" of the Ottoman Empire, further, the Allies granted Italy barely half of the European territories that had been promised-prompting nationalists to label the result as a "Mutilated victory"; that angry sentiment helped to the rise of the fascist dictatorship of Benito Mussolini in 1922.

Roy Pryce summarized the bitter experience:

The Italian leadership was inexperienced, unfamiliar with international affairs, and often quite ill. Forces outside the government played minor roles. The business and financial communities wanted peace, but they were ignored in the decision-making. Likewise intellectuals and foreign policy experts, as well as nationalistic pressure groups, were ignored. The King (since July 1900 it was Victor Emmanuel III) had nominal power over war and peace, but he had severe psychiatric problems in 1914, and in any case he turned over all major issues to his cabinet. Prime Minister Antonio Salandra took office in March 1914, had little experience in foreign affairs, and had no talent or taste for statecraft. The decision for war was in the hands of Foreign Minister Antonio di San Giuliano, an experienced diplomat, cynical and cautious. He was in poor health and died in October 1914. He was replaced by Sidney Sonnino, who maneuvered to join the Allies primarily to gain territory. Tommaso Tittoni, the ambassador to France, was often consulted; he also pled for joining the Allies. Civilian politicians marginalized the generals; the chief of staff died on July 1, and he was finally replaced by General Luigi Cadorna in late July. Cadorna exaggerated the Italian Army's capabilities to the unsuspecting civilians, while working hard to remove its weaknesses. All of the leaders distrusted Austria, and were eager to take control of the Austrian province of Trentino-Alto-Adige in the Alps, and the Austrian city of Trieste. They all distrusted the Ottoman Empire, and were proud that Italy had recently seized control of the Ottoman holdings in Libya. Italy, Austria and Serbia were all contending for control of Albania.