One word: Afghanistan.
The "Northern Distribution Network" runs through Russia. The Pakistan route has been unreliable.
The Administration has said that it would be "grave mistake" for Russia to move on Ukraine. But our interest in keeping the northern supply lies to Afghanistan open will probably surpass any annoyance that we may have over whatever Russia does to Ukraine.
Ukraine isn't bound to us by any treaty that I am aware of. With Yanukovych claiming that he is still the rightful president of Ukraine, if he invites the Russians to act, they may have all the cover that they feel that they need.
UPDATE: In a comment, Snowdog posted a link to a Daily Mail story which claims that the Budapest Memorandum of 1994 obligates the UK and US to go to war over Ukraine. The problem for the Daily Mail is one of reading comprehension. The text of the Budapest Memorandum does no such thing. All it pledges us to do is not use an economic lever against the Ukrainians, to not nuke them, to respect Ukraine's territorial integrity and, in the event of aggression against Ukraine, to trot immediately to the UN Security Council and seek a resolution to do something.
Which, as everyone who signed the Memorandum knew in 1994, and which the Daily Mail could apparently not comprehend now, would mean nothing if the Russians moved into Ukraine. Because the Russians have a permanent seat on the Security Council and have the power to veto anything that is being considered by that august body.
Go read it if you like, and judge for yourself. It's not a long document.
Sweet, Selfish, And Saucy – Just How We Like It!
17 minutes ago