There is no way Putin will go to strategic nuclear over a regional war, he knows what MAD means as well as the rest of us.
His threats can only refer to tactical nukes but he has been warned about that, and I reckon the US electorate will approve of "boots on the ground" to decapitate the Kremlin if he were to use them.
Why would he use one? A tactical nuke's purpose is to kill a concentration of the enemy thus allowing a force ,well equiped with protective gear, to make the decisive breakthrough. [Hitler would have used one in The Bulge if he had one]. On a 1,000 km front, with attacking forces scattered along that length and defenders equally scattered, where could one be used to gain a decisive advantage?
