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In my opinion it's miraculous that Finland remained sovereign even after an invasion by the USSR and it's involvement with the Third Reich. Argue all you want that Finland had rugged terrain and was pretty much what Sweden and Switzerland was to the Nazi's but I believe that the Soviets had the motivation in to incorporate what they had lost after WW1.
If Finland were to come under Soviet control how would it happen? Would it be annexed by the Soviets during the Winter War? Would it be annexed by the Soviets during their counterattack through Europe? Or would it be incorporated into the Eastern bloc after WW2 and be a Soviet puppet?