Italy joined in WWI on May 23rd, 1915 when they declared war on the Austro-Hungarian Empire. Their participation in the war was not however an impromptu action, for months England and France had been bribing Italy with the promise of more territory. England even signed a secret treaty with Italy, The Treaty of London promising them territories such as Dalmatia and Istria. England and France were both acting with the selfish want to open up a new war front south of the western front effectively splitting the central powers and weakening them on both fronts. This plan was logical but dependent on the success of Italy, which they would not find.
The Italians managed to only gain 10 miles of Austrian controlled land by the end of the war, and even with some victories they still experienced catastrophic losses. One such loss was the Battle of Caporetto, which was a series of losses for the Italians that left not only a large psychological impact but also was a point of shame and humiliation. There were some small victories such as the 1918 win at Vittorio Veneto. In reality Italy was not militarily or economically prepared for war, and were greatly overpowered by the Austrian and German troops.
Overall Italy’s involvement in the war did not have the intended impact as the Italians were widely unsuccessful in battle. The number of casualties, and the terrible strain on the economy had inflation rates and unemployment rates higher than ever. In fact in just three years the Italian government had spent more than it had in the 50 years prior only to receive little to no reward at Versaille. The government was humiliated and overshadowed by the big three (America, England, and France), and was even further shamed by not acquiring the promised territories from the unofficial Treaty of London. World War one left Italian nationalists angry and more than a little bit embarrassed.
Interestingly enough Italy should have been fighting on the side of the Triple Alliance, because they were allied with Austro-Hungary and Germany, but they decided to wait and see which side to take as the war progressed. The nationalist party was appalled wanting to join in quickly and readily, the socialist party was not so sure. Mussolini, who would get kicked out of the socialist party, changed his tune from anti-war to pro-war shortly after the war started prompting Italy to join in the great drama instead of remaining simply audience members. Even though the socialist party kicked Mussolini out, and had already taken an anti-war stance, many young socialists began to feel the pull of war. The government joined the war effort for the reasons listed above.