A review of the military situation in Ukraine suggests that Russia could probably conquer Ukraine, but could not occupy it for long.

Russia might attack through Belarus. Or it might try to capture only parts of Eastern Ukraine.

I am skeptical of the idea of deterring that attack by threatening economic sanctions, because that approach has almost never succeeded in changing a country's policies.

This suggests to me that the US and NATO should announce a commitment to defend Ukraine from an overt invasion by Russia, and move divisions into Poland (if Poland agrees) near the frontiers with Ukraine and Belarus.

If Russia attacks Ukraine through Belarus, it would be legitimate and easy to send NATO troops into Belarus to fight the Russian troops there. This would probably spark an uprising, as it would offer an opportunity to overthrow Lukashenko. The Belarusian army would be unable to suppress an uprising while facing invading forces. Its units might even accept the invitation to change sides, or remain in their bases doing nothing.

I think this would make Putin hesitate to launch an invasion. He might engage in destabilization, but Ukraine does not need NATO troops to deal with that.