Wikipedia: Military operations in Italy.

Nominally allied with the Central Powers of the German Empire and the Empire of Austria-Hungary in the Triple Alliance, the Kingdom of Italy refused to join them when the war started in August 1914. In fact, Germany and Austria–Hungary had taken the offensive while the Triple Alliance was supposed to be a defensive alliance. Instead in May 1915, almost a year after the war's commencement, after a period of wavering and after secret negotiations with France and Great Britain in which Italy negotiated for territory if victorious, Italy entered the war on the side of the Allies.
Italy fought mostly against Austria-Hungary along the northern border, including high up in the now-Italian Alps and along the Isonzo river. The Italian army repeatedly attacked Austria, making little progress and suffering heavy losses, and then being routed in 1917 by a German-Austrian counteroffensive after Russia left the war allowing the Central Powers to move reinforcements to the Italian Front from the Eastern Front.
In October 1918 the Italians attacked again. The Austrian army broke, and the Italians drove deep into Austrian territory, leading to the collapse of Austria-Hungary. Fighting ended on 3 November 1918. Italy and the Allies had been victorious. Italian armed forces were also involved in the Western Front and in the Middle Eastern theatre of World War I. At the end of World War I, Italy was recognized a permanent seat in the League of Nations' executive council along with Britain, France and Japan.

More photos...