It is not our responsibility to defend every nation in the world. We do not have a treaty with Ukraine (at least not a public one) and so we do not have a dog in this hunt.
European nations are the ones that should be leading this war (if they actually wanted to defend Ukraine).
So, yeah, I would have no problem allowing Russia to do what ever they think they could get away with.
Now if Russia was to attack a NATO nation that is another story (although I am not sure what we get out of being part of NATO other then paying the bills).
If I remember correctly we were part of an agreement to return all the Ukraine nukes to Russia in exchange for promising to protect Ukraine indepemdemce. I seem to recall that Russia was part of that agreement which they have so vilely ignored.