Russia's willingness to violate Ukraine's territorial sovereignty is the gravest challenge to the European order in over half a century. The conflict pits a vast nuclear power against a state equal in size to France, an autocratic regime against a revolutionary government. The Russian intervention in Ukraine raises questions about the security guarantees that the West made to Ukraine after the country gave up its nuclear weapons in 1994, and it flies in the face of many Europeans' belief that, in recent years, a continental war has become all but impossible. The end result may be the emergence of a...