Ethnic Finns and volunteers from Finland started the East Karelian Uprising in an attempt to wrest East Karelia from the USSR and join it to Finland. While initially successful, the uprising was defeated by the USSR in February, 1922.
One of the Finnish Kindred Nations Wars, the war would have expanded Finland to the White Sea had it been successful and have brought in territory occupied by ethnic Finns, but Finns who had never been part of the Grand Duchy of Finland. It would have been a significant expansion of Finland.
It's an interesting example of nationalist hopes in the post Great War era.
