Estonian Soviet Socialist Republic

The Estonian Soviet Socialist Republic (Estonian SSR, ESSR), also known as Soviet Estonia, was a de facto administrative subunit (union republic) of the Soviet Union, covering the occupied and annexed territory of Estonia in 1940–1941 and 1944–1991. It was nominally established to replace the until then independent Republic of Estonia on 21 July 1940, a month after the 16–17 June 1940 Soviet military invasion and occupation of the country during World War II. After the installation of a Stalinist government which, backed by the occupying Soviet Red Army, declared Estonia a Soviet constituency, the Estonian SSR was subsequently incorporated into the Soviet Union as a union republic on 6 August 1940.

Source: Wikipedia — Estonian Soviet Socialist Republic (CC BY-SA 4.0)