What would America fight for?

Cover story from The Economist, a London based weekly news magazine.  Written from the point of view of Europeans, terrified that the Americans will let the Russki's eat them up.  The article reaches no worthwhile conclusions.  They don't know what the Americans might do.  For that matter I don't think we Americans know what we might do. 
   With the exception of the Brits, the rest of Europe has disarmed, and can offer no more effective military resistance to a Russian invasion than the Ukrainians have.  Plus, the Russians can turn off the heat all over Europe if they please, so the Europeans are timid about economic sanctions.  In actual fact the Russians have more sanctions to lay on the Europeans  than the Europeans have to lay on the Russians.
   So far, the Russians are attacking an non NATO member, and are going to some trouble to disguise their land grab as another Anschluss, (the people really want to become part of Russia).  After the Ukraine aggression has cooled off, say in a year or two, the Russians may try the same thing on a NATO member.  Would the US live up to its treaty obligations and defend NATO members?  The Europeans fear that we won't. 
   And if we don't, the Russians will move on Europe, country by country, and in a few years control everything up to the English Channel.