A country of southeast Europe on the Black Sea. Settled in the 6th century ad by Slavs, it was subject to the Ottoman Empire from the late 14th century until 1908, when it became an independent kingdom. The Soviet Union invaded in 1944 and established the People's Republic of Bulgaria. Communist rule came to an end in 1989, and a democratic constitution was established in 1991. Sofia is the capital and the largest city.