On the 23rd May 1915, Italy entered the First World War on the side of the Triple Entente and declared war on Austria-Hungary. Italy was actually Austria-Hungary’s ally under the terms of the Triple Alliance, but the Italian government had initially opted for neutrality before being persuaded to join with its theoretical opposition. Under the terms of the Triple Alliance, Italy was well within its rights not to provide military assistance to Germany and Austria-Hungary since the treaty was entirely defensive. Since Austria-Hungary had instigated hostilities against Serbia, Italy argued that the alliance was void.
Italy therefore remained neutral for the first nine months of the war. However, behind the scenes Prime Minister Antonio Salandra and his minister of Foreign Affairs, Sidney Sonnino, were investigating which side would be the best to join. In a secret agreement signed on 26th April in London, Italy agreed to leave the Triple Alliance, join the Triple Entente, and declare war on Austria-Hungary and Germany. Assuming they won, Italy would in return receive large areas of territory from the Central Powers such as Italian-populated areas of Austria-Hungary and in the region of the Adriatic Sea.
Italy duly entered the war against Austria-Hungary on 23rd May 1915. Despite superior numbers, the Italians struggled against Austria-Hungarians. However, they did emerge victorious and so Premier Vittorio Emanuele Orlando went as the Italian representative to the Paris Peace Conference. However, the offers of land were not as much as Italy had hoped for and so he left the Conference in a boycott.