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 the UK 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. The UK, France and Italy then declared war on Bulgaria. Although Bulgaria, in alliance with Germany, Austria-Hungary and the Ottomans, won military victories against Serbia and Romania, occupying much of Macedonia (taking Skopje in October), advancing into Greek Macedonia, and taking Dobruja from the Romanians in September 1916, the war soon became unpopular with the majority of Bulgarian people, who suffered great economic hardship and also disliked fighting their fellow Orthodox Christians in alliance with the Muslim Ottomans. The Agrarian Party leader, Aleksandur Stamboliyski, was imprisoned for his opposition to the war. The Russian Revolution of February 1917 had a great effect in Bulgaria, spreading antiwar and anti-monarchist sentiment among the troops and in the cities. In June Radoslavov's government resigned. Mutinies broke out in the army, Stamboliyski was released and a republic was proclaimed.
I didn't know that Bulgaria partecipated in WWI on the Central Powers side. Did they send troops to any of the major fronts? That would make an interesting story (Something like Bulgarian infantry fighting in the Alps).
