There are multiple scenarios. A total nuclear holocaust is possible, sure. But so is a prolonged ground engagement on a scale close to or surpassing WW2 with limited exchanges of nuclear weapons.
I think the use of nuclear attacks of any kind are off the table outside of a last ditch ploy to outright win through annihilation. Anything else could easily spin into a total nuclear apocalypse, unless you think in that case world leaders would be able to hold their trigger fingers and retaliating to a nuclear strike on their soil.