Yeah, Finland was part of Tsarist Russia. On revolution it had a civil war, whites won after significant support from Germany. And Finnish reds were driven into exile or hiding. Finland interwar kept its left suppressed and there was an attempt to purge all the communists by a right wing proto Nazi group. Tensions with USSR were high and their were multiple tit for tat raids. The agreed upon border was the old Grand Duchy's border and was popular with either the Soviets or Finnish Nationalists because a) the Border was dangerously close to Leningrad, at less the 30KM and b) it kept Keralia in Soviet hands. In 1939, Soviets following Nazi Soviet Pact requested Finland exchange a strip of territory around Leningrad and basing rights in the Gulf of Finland in exchange for Soviets giving up an area twice its size in Keralia. In terms of land gained Finland was getting a good deal but Finland declined. Which pissed ussr off and increased tensions at a time when both sides were tense.
Soviets cynically used WW2's outbreak to settle the issue and invaded Finland. Finland and Soviets fought the winter war with Finns getting aid from both Nazi Germany and West but west unwilling to declare war on USSR because everyone saw it'd be dumb. Soviet high command was not prepared for war in the terrain of the Finnish border and was due to the purging of Mikhail Tukhachevsky were in middle of Reorganisation on a doctrine level so were first very ineffective and it took til like March 1940 and an awful lot of casualties to finally break Finnish defences. Soviets and Finns immediately did a deal giving Soviets all of what they wanted plus Vyborg. which I think shows Stalin wasn't trying to conquer anything he just cynical wanted to settle the border issue. Why do the deal when the Finnish front is broken?
After Barbarossa, Finns joined Germans to try to get the land back and they then besieged Leningrad, Finns weren't in the axis but were extremely close to the Nazis as working alongside each other as allies against the USSR (they're not just co-beligerants as Libs try to claim), they did this because they were genuinely sympathetic to Germany's anti Bolshevik plan and saw it as a cynical attempt to get their land back with German aid.
This didn't go so well and by 1944 they were pushed back to the 1940 Armistice line. Finland then did another deal with Soviets; where Finland was basically let off Scott free if they they expelled all the Germans and ceded the port of Potsamo and let Soviets have a Naval base on the gulf of Finland, ie. Exactly what the Soviets asked for in 1939.
After the war Finland chose to become actively neutral. in regards to USSR, It proclaimed it self as independent and a champion of Liberal democracy but at the same time willing to work with Soviets and would sign a mutual assistance pact to show it wasn't a threat. In, regards to west it said it sympathised with them politically and opened its markets to America trade but would not fight the Soviets so It deliberately distanced itself from NATO. The point was to say it was neither a threat to either. To which the Soviets more or less left her alone because it realised she posed no real threat while Americans got access to her trade so was happy that Soviets didn't control it.