The simple answer: to gain territory.

When war broke out, Italy was indeed allied to its old enemy Austria-Hungary along with the German Empire. Baron Sonnino, Minister of Foreign Affairs - under Prime Minister Salandra - with a small minority were sympathetic to the Central Powers. He assumed that Germany, with their economic clout and vast manpower, would win the war. However, most Italians were happy with being neutral.

Salandra and Sonnino were unable to receive Austro-Hungarian agreement for expansion of Italian territories in the Adriatic. They began fishing with the Entente Powers. France, the United Kingdom and Russia were only too happy to fulfil the Irredentismo italiano in the secret Treaty of London of 1915. Italy was promised Trieste, Tyrol, Austrian Littoral, Dalmatia, ports in Albania; German colonies in Asia and Africa along with a protectorate over Albania, Dodecanese Islands and a share of the Ottoman Empire if it would partitioned.

As Sonnino said himself:

If a lasting peace is to be assured, it is necessary that Italy obtain secure national frontiers - an indispensable condition for her full independence.

To Sonnino, the Treaty of London was a solemn bargain but by the 1919 Peace Conference, it was an embarrassment to the French and British. President Wilson stated that the United States was not bound by secret deals. At the Peace Conference, the British and French tried to wriggle out of their agreement by stating that Italy had no fully committed to the Treaty: Italy had delayed their attack on Austria-Hungary and Regia Marina had not patrolled the Mediterranean or Adriatic as promised. Indeed, monies given to Italy were not spent on the war effort.

By 1919, the attitude to Italy as reported by Lord Stanley, British Ambassador to France, to his superior, Arthur Balfour, Foreign Secretary:

"Their attitude has been one of supreme contempt up to now and now it is one of extreme annoyance. They all say that the signal for an armistice was a the signal for Italy to begin to fight."

Eventually Italy crashed out of the Peace Conference because of general disappointment towards the Entente Powers. The Italians felt wronged by the vittoria mutilata (mutilated victory) to quote the famous Italian poet, Gabriele D'Annunzio. It fuelled the rhetoric of Italian nationalism during the Inter-war years and it was a corner stone of Mussolini's propaganda.

Main source: Macmillan M., 2002,. Peacemakers: six month that changed the world, J. Murray Ltd., London