Italy Declares War on Bulgaria
In the aftermath of the Balkan Wars, Bulgarian opinion turned against Russia and the western powers, whom the Bulgarians felt had done nothing to help them.
The government of Vasil Radoslavov aligned Bulgaria with Germany and Austria-Hungary, even though this meant also becoming an ally of the Ottomans, Bulgaria's traditional enemy. But Bulgaria now had no claims against the Ottomans, whereas Serbia, Greece and Romania (allies of Britain and France) were all in possession of lands perceived in Bulgaria as Bulgarian. Bulgaria, recuperating from the Balkan Wars, sat out the first year of World War I, but when Germany promised to restore the boundaries of the Treaty of San Stefano, Bulgaria, which had the largest army in the Balkans, declared war on Serbia in October 1915. Britain, France and Italy then declared war on Bulgaria.
In spite of its official status as member of the Triple Alliance together with Germany and Austria-Hungary, in the years before the outbreak of the conflict the Italian government had enhanced its diplomatic efforts towards United Kingdom and France. This was because the Italian government had grown convinced that a support of Austria (which had been also the traditional enemy of Italy during the 19th century Risorgimento) would not grant to Italy the lands the country was aiming for in its territorial expansion: Trieste, Istria, Zara and Dalmatia, all Austrian possessions. In fact, a secret agreement signed with France in 1902 practically nullified Italy's membership in the Triple Alliance.
Wikipedia: Bulgaria During World War I
Wikipedia: Italy In World War I