On 17 September 1939, early in the morning, the Soviet Union invaded Poland. Poland was already in a state of war with Nazi Germany from the 1 September 1939. The Soviet invasion of Poland was a direct result of the Ribbentrop-Molotov Pact, signed between Nazi Germany and the Soviet Union on 23 August: a secret protocol that cut the continent into two spheres of influence, split between two totalitarian systems – that of Nazi Germany and that of Soviet Union. At the end of September 1939 the division of Poland was confirmed by German–Soviet Treaty of Friendship, Cooperation and Demarcation which included a correction of the borders first drawn in the secret clause of the Ribbentrop-Molotov Pact. It was the beginning of a 2-year long occupation of Central Europe by two totalitarian regimes.
Andrzej Włusek: Soviet invasion of Poland 1939