If NATO allows one of its member states to be invaded without responding, then NATO is a dead letter, not just for the Baltic States but for the whole 29-member alliance. Collective security is the only way to make an aggressor behave. Want to avoid a war? Don't give an inch of an ally's territorial integrity to a hostile power. Especially if you expect collective allies come together when that hostile power tangles with you.
What would be the effect on the European countries themselves if NATO is allowed to collapse, and Europe goes back to being the sort of neighborhood where sovereign states and Russia routinely invade each other and take bites out of each other's territories?
So, we need to make 100% crystal clear to Vladimir Putin that:
An armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain security.
There is no asterisk after "one or more of them" except for the countries on the list in the Secret Protocol of the Putin-Trump Pact. Unfortunately, Trump (aided and abetted by the oh-so-uber-patriotic Republican Party) has already dangerously weakened the collective institutions and treaty's the United States and the world depend upon for their peace and security.