The ESSR was initially established on the territory of the Republic of Estonia on July 21 , 1940 , following the invasion of Soviet troops on June 17 , 1940 and the installation of a puppet government backed by the Soviet Union , which declared Estonia a Soviet state .
The ESSR replaced the Republic of Estonia on 21 July 1940 , after the invasion of Soviet troops on 17 June 1940 . A puppet government backed by the Soviet Union , declared Estonia to be a Soviet state .