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.
Source: Wikipedia — History of Bulgaria (1878–1946) (CC BY-SA 4.0)