In such a hypothetical scenario Ukraine becomes part of Russia, and nothing really changes for the rest of the world. Nobody has any obligations to rescue Ukraine. Sure, we can start a war to reclaim Ukraine's territory, but we're not going to. Likewise, Putin won't attack Europe, because Russia can't afford such a large-scale conflict. Especially one that would likely end with their loss.

We haven't even started with the Baltics, baby-cakes