[lumost]

When the US and Soviet Union established MAD the prevailing method of fighting wars was to fire/drop/shoot as many low-accuracy munitions as possible at the enemy. More ordinance was expended in the Korean war than in WW2, and by the Vietnam war planners had to moderate B-52 carpet bombing as 12x B-52s carry nearly a nukes worth of ordinance. North Korea lost roughly ~20% of its population in a conventional war.

It's easy to justify nuclear escalation when fighting this form of total war, and difficult to when the only casualties are military and the bombs mostly fall on equipment. While a land invasion or surprise strike of the continental US/EU/China/Russia would surely trigger a nuclear response, a direct battle over territories/international policy wouldn't be as clear cut.