the Kinship wars were a sad chapter in our history, at one point Finnish government unofficially supported the invasion while the Karelians didn't want Finns there, then when at another point at least part of Karelia wanted to separate from Soviet Russia with Finnish help (some wanting independence while others apparently thought it would be better to join Finland, at least that's what I have read...) & the right-wing radicals of Finland wanted to help the government opposed the idea, then after the wars Stalin came to power & being his lovely paranoid self wasn't exactly kind to the Finnic groups on the North-Western frontier of USSR, then WWII did even more damage (lots of native Karelian speakers dead, the dialect continuum between Finnish and Karelian got severed by the evacuation of Finnish Karelians from Ladoga Karelia and Karelian Isthmus & Stalin's decision to populate the now depopulated area with Russian-speakers) & now Karelian language is in danger of dying out...
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