Would withdrawal alone be a loss, or would it have to be withdrawal with a full status quo ante situation wrt Crimea's status (not leaving a separatist puppet government, et al)? i'm just trying to sort out what we can realistically expect/effect.
What do you mean, specifically, about action in Europe that could have prevented this?
I think withdrawal alone would be a loss because it's a tacit admission of over-calculation on Moscow's part. If said situation were to play out Putin would obviously claim the reason Russia pulled out was because they felt as though the interests of the people they went in to protect had been adequately safeguarded.
After that, I don't support the United States being a direct part of any nation building effort in Ukraine (or the region). We leave that to the EU and, more specifically, Eastern European states like Poland and Hungary. Russia will obviously be involved, too, and rightfully so, because they do share a border with Ukraine. Emphasis on border.
As for what 'specific' actions could've prevented this? That would be impossible to pinpoint. But I feel comfortable asserting that the Obama Administration's complete lack of involvement in European affairs certainly precipitated Putin's willingness to plow 6,000 troops into Ukraine just a few hours after Obama specifically warned him against doing just that.