Although a member of the Triple Alliance, Italy did not join the Central Powers – Germany and Austria-Hungary – when World War I started on 28 July 1914. In fact, those two countries had taken the offensive while the Triple Alliance was supposed to be a defensive alliance. Moreover the Triple Alliance recognized that both Italy and Austria-Hungary were interested in the Balkans and required both to consult each other before changing the status quo and to provide compensation for whatever advantage in that area: Austria-Hungary did consult Germany but not Italy before issuing the ultimatum to Serbia, and refused any compensation before the end of the war.

Almost a year after the war's commencement, after secret parallel negotiations with both sides (with the Allies in which Italy negotiated for territory if victorious, and with the Central Powers to gain territory if neutral) Italy entered the war on the side of the Allied Powers. Italy began to fight against Austria-Hungary along the northern border, including high up in the now-Italian Alps with very cold winters and along the Isonzo river. The Italian army repeatedly attacked and, despite winning a majority of the battles, suffered heavy losses and made little progress as the mountainous terrain favoured the defender. Italy was then forced to retreat in 1917 by a German-Austrian counteroffensive at the Battle of Caporetto after Russia left the war, allowing the Central Powers to move reinforcements to the Italian Front from the Eastern Front.

The offensive of the Central Powers was stopped by Italy at the Battle of Monte Grappa in November 1917 and the Battle of the Piave River in May 1918. Italy took part in the Second Battle of the Marne and the subsequent Hundred Days Offensive in the Western Front. On 24 October 1918 the Italians, despite being outnumbered, breached the Austrian line in Vittorio Veneto and caused the collapse of the centuries-old Habsburg Empire. Italy recovered the territory lost after the fighting at Caporetto in November the previous year and moved into Trento and South Tyrol. Fighting ended on 4 November 1918. Italian armed forces were also involved in the African theatre, the Balkan theatre, the Middle Eastern theatre and then took part in the Occupation of Constantinople. At the end of World War I, Italy was recognized with a permanent seat in the League of Nations' executive council along with Britain, France and Japan.