History of Bulgaria (1878–1946)

After the Russo-Turkish War of 1877–1878, the 1878 Treaty of Berlin set up an autonomous state, the Principality of Bulgaria, within the Ottoman Empire. Although remaining under Ottoman sovereignty, it functioned independently, taking Alexander of Battenberg as its first prince in 1879. In 1885 Alexander took control of the still-Ottoman Eastern Rumelia, officially under a personal union. Following Prince Alexander's abdication (1886), a Bulgarian Assembly elected Ferdinand I as prince in 1887. Full independence from Ottoman control was declared in 1908.

In the 1912–1913 Balkan Wars, Bulgaria initially formed an alliance with Greece, Serbia and Montenegro against the Ottoman Empire, and together they conquered a great deal of Ottoman territory. Bulgaria, however, unhappy with the resulting division of territory, soon went to war against its former allies Serbia and Greece and lost territory it had gained in the first war. The First World War (1914–1918) saw Bulgaria fighting (1915–1918) alongside Germany, Austria-Hungary, and the Ottoman Empire. Defeat led to the Treaty of Neuilly-sur-Seine (27 November 1919), in accordance with which Bulgaria lost further territory. Social problems and political instability persisted throughout the Interwar period. In the Second World War (1939–1945), Bulgaria again allied with Germany (March 1941). Although Sofia attempted to pull out of the war as the Soviet Union advanced towards its territory (1944), the Red Army invaded (September 1944), and a communist government came to power (1944–1946) and established the People's Republic of Bulgaria (1946–1990).