Evacuation of Finnish Karelia
As a result of the 1940 Moscow Peace Treaty that concluded the Winter War, Finland ceded a portion of Finnish Karelia along with other territories to the Soviet Union. As a result, about 410,000 people, or 12% of Finland's population, were relocated to the remaining parts of Finland.
Source: Wikipedia — Evacuation of Finnish Karelia (CC BY-SA 4.0)