Ah, but there's the rub. If you have nukes and your opponent has nukes, and your opponent tells you they will use their nukes if you use yours, when do you use your nukes?

Probably to prevent core territorial incursion to the extent it would dissolve you as a viable entity. But to protect / secure the South China Sea? To win a proxy war in the Middle East? Where's the line?

Because it's certainly not "as soon as hostilities break out."