Why did Italy join the Allies in World War I?

Although Italy was initially a member of the Triple Alliance with Germany and Austria-Hungary, it remained neutral when World War I began in 1914. Italy ultimately joined the Allies in May 1915 primarily due to the prospect of significant territorial expansion.123

While the Austro-Hungarian government offered to cede the territory of Trentino in exchange for Italian neutrality, the Triple Entente provided a more lucrative proposal. Through the secret Treaty of London signed in April 1915, the Allies promised Italy not only Trentino but also South Tirol, Trieste, Gorizia, Istria, and northern Dalmatia.2

This decision was also driven by the influence of Irredentist patriots who sought to "redeem" Italian-inhabited lands still under foreign rule, particularly those controlled by Austria-Hungary. Despite significant domestic opposition from neutralists, the Italian government pushed for intervention, viewing the war as a means to complete national unification and secure regional dominance.32