I have a feeling that in the next few weeks the EU and a US-less NATO force will find themselves in the midst of a pretty bloody fight. Finally they get to try to be the world's police and we (the US) have the opportunity to scrutinize and play Monday morning quarterback.
I have a very strong feeling that the US will definitely sit this one out (at least until it gets out of hand). The EU is already in the process of deploying 2,000 troops to the border of Kosovo and Serbia.
I think the most we will do in the event of a war will be to send money and weapons. I can fathom another US battle front opening up unless it's with Iran.