<P> The occupation of the Baltic states was the military occupation of the three Baltic states -- Estonia , Latvia and Lithuania -- by the Soviet Union under the auspices of the Molotov -- Ribbentrop Pact on 14 June 1940 followed by their incorporation into the USSR as constituent republics , unrecognised by most Western powers . On 22 June 1941 , Nazi Germany attacked the USSR and within weeks occupied the Baltic territories . In July 1941 , the Baltic territory was incorporated into the Reichskommissariat Ostland of the Third Reich . As a result of the Baltic Offensive of 1944 , the Soviet Union recaptured most of the Baltic states and trapped the remaining German forces in the Courland pocket until their formal surrender in May 1945 . The Soviet `` annexation occupation '' ( Annexionsbesetzung or occupation sui generis ) of the Baltic states lasted until August 1991 , when the Baltic states regained independence . </P>
when did the soviet union first gain control of parts of poland and the baltic republics
14 June 1940