The Great Patriotic War, Act I

Wikipedia:

The Soviet invasion of Poland was a military operation by the Soviet Union without a formal declaration of war. On 17 September 1939, the Soviet Union invaded Poland from the east, sixteen days after Germany invaded Poland from the west. Subsequent military operations lasted for the following 20 days and ended on 6 October 1939 with the two-way division and annexation of the entire territory of the Second Polish Republic by Nazi Germany and the Soviet Union. The Soviet (as well as German) invasion of Poland was indirectly indicated in the „secret additional protocol“ of the Molotov–Ribbentrop Pact signed on 23 August 1939, which approximately divided Poland into „spheres of influence“ of the two powers and questioned future existence of the Polish state.