 The Soviet Union and Nazi Germany signed the Molotov – Ribbentrop Pact in August 1939 . The pact was nominally a non @-@ aggression treaty , but it included a secret protocol in which the Eastern European countries were divided into spheres of interest . Finland fell into the Soviet sphere . On 1 September 1939 , Germany began its invasion of Poland and two days later Great Britain and France declared war against Germany . Shortly afterwards ( 17 September 1939 ) , the Soviets invaded eastern Poland . The Baltic states were later forced to accept treaties allowing the Soviets to establish military bases and to station troops on their soil . The government of Estonia accepted the ultimatum , signing the corresponding agreement in September . Latvia and Lithuania followed in October . Unlike the Baltic states , Finland started a gradual mobilisation under the guise of " additional refresher training . " 
