Ranar 1 ga Satumba, 1939, Nazi Jamus ta mamaye Poland kuma ta fara Yaƙin Duniya na II. A ranar 17 ga Satumba, 1939, Tarayyar Soviet, bisa ga yarjejeniyar sirri na Yarjejeniyar Molotov-Ribbentrop, ta mamaye Poland daga gabas. A cikin 'yan watanni, ƙasar Soviet ta kai wa ƙasar Finland hari
Ka yi haƙuri. Sakamakon wannan yaƙin, an tilasta wa Finland ta ba da ƙasa, gami da sassan lardin Viipuri, ga Tarayyar Soviet a cikin Yarjejeniyar Zaman Lafiya ta Moscow a farkon 1940. Finland ta rasa iyakokinta na asali tare da Kogin Rajajoki () a kudu. 22,973 km2, ko kashi 71.5 na lardin a kan Karelian Isthmus, gami da biranen Viipuri da Sortavala, sun zama wani ɓangare na sabon kafa Karelo-Finnish SSR a cikin Tarayyar Soviet. Bayan yarjejeniyar zaman lafiya, an kwashe dukkan mazaunan yankunan da aka ba su, sama da mutane dubu dari huɗu, zuwa tsakiyar Finland.