Ah. I was beaten to the punch by USEC, who thoroughly already explained the Triple Alliance. About the only thing I can add is the official excuse by Italy why they didn't honor the treaty: they claimed it was a defensive agreement, but that Austria-Hungary had taken the offensive in Serbia.
Though, actually, Italian honoring the war on the side of the Triple Alliance would have been a disaster for Italy. If you look at their historical capabilities against Austro-Hungary in the Austrian Alps, multiply those problems manifold against a more competent enemy through the French Alps, which are both a shorter and more geographically-hostile front. Famously, in very next World War, Mussolini's harebrained attempt to invade France via these lands ended in a complete and utter disaster, even while Germans were actively and successfully taking Belgium and Paris. Italy was also completely reliant on economic imports from Britain; it was warned even during the Triple Alliance that while Italy would honor it against France, they would not be able to do so against the UK as to do so would spell disaster for the Italian economy. One can't even argue greater Italian competence in the First World War; the commander in chief through most of the war was Cadorna, who is either most famous for spending the opening of the Caporetto Offensive on leave, for his orders to execute the officers of any unit that retreated, or bringing a cumulative 6% of the entire Italian Army up on disciplinary charges at one time or another. The only place where it would have changed matters is in Africa, where you would likely see a British- and French-supported Senussi campaign supported from Egypt and Algeria, in much the same way the British stirred up Arab nationalism in Arabia for their own ends against the Turks (or, for that matter, how the Germans and Turks supported the Senussi against the Italians and British, but without the disadvantages those powers had in reaching inner Libya). The war would likely end in a Mandatory Cyrenaica and Tripolitania to join Mandatory Syria, Palestine, Transjordan, and the others, and absolutely no territorial concessions from Austria or Entente ally Yugoslavia.