One issue is that Lithuania does not share a border with the Russian “mainland” but with Belarus, independent from Russia but closely aligned with Moscow. Assuming Russia has the resources to threaten or invade Lithuania, they would need to have the cooperation of Belarus.
This is true. If Belarus was to cooperate by giving free passage through for Russian troops then they would be culpable in the ensuing war and would not be immune from military action. Recall that they already gave free passage in February to Russian troops to invade Ukraine. Belarus might try to argue that they had no choice but that would ring rather hollow.