On September 1, 1939, Nazi Germany invaded Poland and started World War II. On September 17, 1939, the USSR, in accordance with the secret protocols of the Molotov–Ribbentrop Pact, invaded Poland from the east. Within months, the Soviet Union launched a war against Finland. As a result of this war, Finland was forced to cede territory, including parts of Viipuri Province, to the Soviet Union in the Moscow Peace Treaty in early 1940. Finland lost its natural border along the Rajajoki River () in the south. 22,973 km2, or 71.5 percent of the province on the Karelian Isthmus, including the cities of Viipuri and Sortavala, became part of the newly established Karelo-Finnish SSR in the Soviet Union. Following the peace treaty, the entire population of the ceded territories, more than four hundred thousand people, was evacuated to central Finland.