Russia 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.
Faced with Russia, Austria–Hungary could spare only one-third of its army to attack Serbia. After suffering heavy losses, the Austrians briefly occupied the Serbian capital, Belgrade. A Serbian counter attack in the battle of Kolubara, however, succeeded in driving them from the country by the end of 1914. For the first ten months of 1915, Austria–Hungary used most of its military reserves to fight Italy. German and Austro–Hungarian diplomats, however, scored a coup by persuading Bulgaria to join in attacking Serbia. The Austro–Hungarian provinces of Slovenia, Croatia and Bosnia provided troops for Austria–Hungary, invading Serbia as well as fighting Russia and Italy. Montenegro allied itself with Serbia.
Wikipedia: World War I
Wikipedia: Bulgaria During World War I