The First World War (1914–1918) was an unexpected development that forced the decision whether or not to honor the alliance with Germany. At first Italy remained neutral, saying that the Triple Alliance was only for defensive purposes. Public opinion in Italy was sharply divided, with Catholics and socialists recommending peace. However extreme nationalists saw their opportunity to gain their "irredenta" – that is, the border regions that were controlled by Austria. The nationalists won out, and in April 1915, the Italian government secretly agreed to the London Pact. Italy would declare war on the Austro-Hungarian Empire in exchange for promises of major territorial rewards. Italy entered the war with an army of 875,000 men, but the army was poorly led and lacked heavy artillery and machine guns, their war supplies having been largely depleted in the war of 1911-12 against Turkey. Italy proved unable to prosecute the war effectively, as fighting raged for three years on a very narrow front along the Isonzo River, where the Austrians held the high ground. In 1916, Italy declared war on Germany, which provided significant aid to the Austrians. Some 650,000 Italian soldiers died and 950,000 were wounded, while the economy required large-scale Allied funding to survive.
Italy blocked serious peace negotiations, staying in the war primarily to gain new territory to the north. The Treaty of St. Germain awarded the victorious Italian nation the Southern half of the County of Tyrol, Trieste, Istria, and the city of Zadar. Italy did not receive other territories promised by the Pact of London, so this victory was considered "mutilated". Subsequently, after the Greco-Turkish War of 1919-1922, Italy formally annexed the Dodecanese (Possedimenti Italiani dell'Egeo), that she had occupied during the war.