I think America has two primary interests in the current situation in Europe. Here is how I would rank them.

1. That no more players enter the war as full belligerents. This limits costs to our nation and preserves our array of options in the end.  The war very likely can and will result in costs to our NATO allies. We have an interest in the war not becoming more politically complex or in raising the stakes. If somehow Poland got sucked into the conflict, we would be faced with incredibly difficult, painful, and immediate choices in the most stressful moment. We don’t want this.
2. That Vladimir Putin’s regime suffer costs and penalties. This is a deterrent against others who would revise or reverse American security arrangements. While we never let Ukraine into NATO, the amount of military assistance and collaboration it was getting from NATO powers made it a kind of NATO ally, or adjunct. Aggressive wars of choice, even ones that don’t touch our core national-security interests, still have costs for us, and it is still worth making them costly for others to pursue. 

These two interests are in obvious tension. One can pursue either of them in such a way that it makes the other harder to achieve. In the last week, I think the mood of the policy hivemind swung towards No. 2 so strongly that it weakened our pursuit of No. 1. And now the ground may be shifting again.