“There are many animals which have what are called dominance contests. They rush at each other with horns—trying to knock each other down, not gore each other. They fight with their paws—with claws sheathed. But why with their claws sheathed? Surely, if they used their claws, they would stand a better chance of winning? But then their enemy might unsheathe their claws as well, and instead of resolving the dominance contest with a winner and a loser, both of them might be severely hurt.” -Professor Quirell
Or to be more explicit, anything less than total war is a dominance contest between factions, not a no-holds-barred attempt to win. Nuclear weapons are useful for deterrence, but if there is a situation in which neither side is willing to simply back down, but both also want to limit the destruction, then a conventional military becomes very helpful.
“There are many animals which have what are called dominance contests. They rush at each other with horns—trying to knock each other down, not gore each other. They fight with their paws—with claws sheathed. But why with their claws sheathed? Surely, if they used their claws, they would stand a better chance of winning? But then their enemy might unsheathe their claws as well, and instead of resolving the dominance contest with a winner and a loser, both of them might be severely hurt.” -Professor Quirell
Or to be more explicit, anything less than total war is a dominance contest between factions, not a no-holds-barred attempt to win. Nuclear weapons are useful for deterrence, but if there is a situation in which neither side is willing to simply back down, but both also want to limit the destruction, then a conventional military becomes very helpful.