https://en.wikipedia.org/wiki/Estonian_Soviet_Socialist_Republic
In response, the Estonian SSR was formally incorporated into the Soviet Union on 9 August 1940, and nominally became the 16th constituent republic of the USSR. (On 16 July 1956, the Karelo-Finnish SSR was demoted to the Karelian ASSR ; from then on until 1991, the Estonian SSR was considered the 15th constituent republic.)