One by one the lines have all been crossed. Patriot missile-defence systems. Bradley armoured fighting vehicles. Leopard and Abrams tanks. Weapons that NATO had previously balked at providing Ukraine, for fear of “provoking” Russia, are now being supplied in abundance, if not in haste. The latest Ukrainian request to give NATO pause is for F-16 fighter jets, though France and Italy are already talking of going ahead with these. That is in part a measure of Russian weakness. Each new weapons shipment brings forth blood-curdling new threats from the Russian side, up to and including nuclear war. But Western strategists...