The Soviet invasion of Poland was a Soviet military operation that started without a formal declaration of war on 17 September 1939. On that morning, 16 days after Nazi Germany invaded Poland from the west, the Soviet Union invaded Poland from the east. The invasion and battle 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 both Germany and the Soviet Union.