[orwin]

Well, I'm pretty sure Europe is capable of sending nuclear missiles that would destroy any FOB Russia set in Finland (this wouldn't trigger Russia doctrine of only using nukes to defend its territory, not its troops).

Invasion would become way to costly for a demographically challenged country.

[fsloth]

Would EU countries waste their nukes in attrition of RU troops on aligned soil? There aren't that many (by official figures). I'm guessing most countries would prefer not to escalate to nukes unless their own territory was threatened.