Although a member of the Triple Alliance, Italy did not join Germany and Austria-Hungary when the conflict started in 1914, on the ground that war was initiated by the two Central Powers while the Triple Alliance was a defensive bloc. Italians protested for the lack of consultation before Austria issued the ultimatum to Serbia and invoked a clause of the Triple Alliance, according to which both Italy and Austria-Hungary were interested in the Balkans and whoever changed the status quo in the region had to compensate the other; Austria-Hungary refused any compensation before the end of the war. In May 1915, after secret parallel negotiations with both sides, the Italians entered the war as one of the Allied Powers, hoping to acquire the Italian-speaking "irredent lands" of Trento and Trieste (in Italian discourse the conflict was described as the "fourth war of independence" against Austria) and other territories (German-speaking South Tyrol, the largely Slavic-speaking regions of Istria and Dalmatia where Italians lived in coastal cities, some colonial compensations) promised them by the allies in the 1915 treaty of London.
Italy opened a front against Austria-Hungary along the Eastern Alps and the Isonzo river. Fighting was marked by trench warfare and attrition. On the Julian sector, the Italian army launched numerous offensives and made several conquests (most significantly Gorizia in 1916 and Bainsizza in 1917), but both sides suffered heavy casualties. On the Asiago plateau, in 1916, an Austrian offensive was followed by an Italian counter-offensive. Italy was forced to retreat in 1917 by a German-Austrian offensive at the Battle of Caporetto, after the Russian collapse allowed the Central Powers to move reinforcements to the Italian Front from the Eastern Front.
