Actually, a diplomatic - read not involving direct hostilities between NATO and Russian forces - is still possible, but only if the US and Europe, i.e., NATO, make it clear that such direct hostilities will happen if the Russians take too many more steps to acquire territory in Ukraine.
That, of course, means that with the pusillanimous laughingstock we have as president there will not be a diplomatic solution, there will merely be acquiesence in what Putin decides to take, including the additional territory he's already sizing up for seizure.