It seems to me that the odds look so grave to you because you gloss over several steps during this potential escalation.

Have a look at this: http://www.zerohedge.com/news/2014-02-28/ukraine-acting-president-says-russia-starts-aggression-against-country-russian-plane Original source: http://www.pravda.com.ua/rus/news/2014/02/28/7016674/

Posturing or not, if this info checks out then by existing treaties USA and UK are obliged to help Ukraine against Russia. You see, Ukraine gave up nukes in exchange for safety guarantees from Russia, UK and USA.

http://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine

Before voting on accession, Ukraine demanded from Russia, the USA, France and the United Kingdom a written statement that these powers undertook to extend the security guarantees to Ukraine. Instead security assurances to Ukraine (Ukraine published the documents as guarantees given to Ukraine[5]) were given on 5 December 1994 at a formal ceremony in Budapest (known as the Budapest Memorandum on Security Assurances[6]), may be summarized as follows: Russia, the UK and the USA undertake to respect Ukraine's borders in accordance with the principles of the 1975 CSCE Final Act, to abstain from the use or threat of force against Ukraine, to support Ukraine where an attempt is made to place pressure on it by economic coercion, and to bring any incident of aggression by a nuclear power before the UN Security Council.

Of course more likely then not we'll find out once again that treaties and words aren't worth anything unless you have the upper hand... but this looks scary enough to me.