Given that Turkey is a NATO member, the idea of US-backed Kurdish guerillas fighting the Turkish army is fascinating - it could be interpreted as an attack by a NATO member on another NATO member, which would presumably give NATO's lawyers something to argue about. How serious is NATO's all-for-one stance? I've always assumed that if push came to shove and the United States ended up fighting Russia, Britain would join in, the rest would either pretend to join in or just refuse.
My hunch is that any conflict would be as a result of overconfidence on the part of President Putin and his aides. A full-on land invasion of Ukraine unsubtly masquerading as an international stability force for example. Assuming it progressed beyond a small skirmish it would be economically ruinous for Russia and would probably end with Putin's retirement to a yacht somewhere.