Both the USSR and Finland figured the fight would be between the great powers and the USSR. The USSR wanted Finland to be neutral to close off that route of invasion, and Finland wanted to be neutral and sit out any war.

https://en.wikipedia.org/wiki/Finno-Soviet_Treaty_of_1948

Under the treaty, which was signed on 6 April 1948, the Soviets sought to deter Western or Allied Powers from attacking the Soviet Union through Finnish territory,

Under the pact, Finland was obliged to resist armed attacks by "Germany or its allies" (in reality interpreted as "the United States and allies")[citation needed] against Finland, or against the Soviet Union through Finland. If necessary, Finland was to ask for Soviet military aid to do so.

The agreement also recognized Finland's desire to remain outside great-power conflicts, allowing the country to adopt a policy of neutrality in the Cold War.

Finland followed the Eastern Bloc countries' decision and did not participate in the Marshall Plan.

But that doesn't hold in today's world where Russia is little more that a terrorist country that wants bits of land, so they joined NATO.

Finland is ready for the invasion - assuming Russia doesn't have any technology developed since 1940