On April 26, 1915, after receiving the promise of significant territorial gains, Italy signs the Treaty of London, committing itself to enter World War I on the side of the Allies.
With the threat of imminent war looming in July 1914, the Italian army under Chief of Staff Luigi Cadorna had begun preparing for war against France, according to Italy's membership in the Triple Alliance with Germany and Austria-Hungary. Under the terms of that agreement, however, Italy was only bound to defend its allies if one of them was attacked first. Italian Prime Minister Antonio Salandra deemed the Austro-Hungarian ultimatum to Serbia late that month an act of aggression, declaring that Italy was free of its alliance obligations, and was officially neutral. In the first year of war, both sides—the Central Powers and the Entente, as the British-French-Russian axis was known—attempted to recruit neutral countries including Italy, Bulgaria, Romania and Greece, to join the war on their side. Italy, more than any other country, was clear about its aims for joining the war effort: to gain the most possible territory for itself and raise its status from a minor to a great power.
In reality, Italy's geographical position—bounded on all sides by the sea, and thus subject to pressure from Britain's great navy—inclined it to favor the Entente. Moreover, past interactions between Italy and Austria-Hungary had been driven more by mutual animosity than alliance, as the Italians had been forced to push the Austrians out of their peninsula in order to achieve unification in 1860. In making a bid for Italy's allegiance in World War I, the Central Powers clashed over Germany's desire to promise the Italians the Trentino region (now occupied by Austria) in return for their entrance into the war. Though Austria-Hungary agreed to cede the Trentino in March 1915, their army's sorry performance against Russia gave the Italians more bargaining power and led them to demand even more territory.
The Entente, for its part, offered much more substantial gains of territory—most of which currently fell within the Austro-Hungarian Empire—and it was under these terms that Italy signed the Treaty of London on April 26, 1915. Italy was promised the fulfillment of its national dream: control over territory on its border with Austria-Hungary stretching from Trentino through the South Tyrol to Trieste. In the treaty, the Allies gave them that and more, including parts of Dalmatia and numerous islands along Austria-Hungary's Adriatic coast; the Albanian port city of Vlore (Italian: Valona) and a central protectorate in Albania; and territory from the Ottoman Empire.
Carrying out its part of the bargain, Italy declared war on Austria-Hungary (but not on Germany) on May 23. The Allies seemingly faced a more difficult task in the fulfillment of their own obligations: another secret treaty, signed March 20, had promised Russia control of Constantinople and the Dardanelles. Both treaties depended on an Allied victory at the Gallipoli Peninsula for their promised gains, which at this point seemed in no way secure. A naval attack against the Dardanelles on March 18 had failed miserably; a massive Anglo-French land invasion, begun the day before the Treaty of London was signed, would soon be stymied by a stiff Turkish resistance.