would you invade your neighbor if it meant they would nuke you capital and largest population centers. its called mutually assured destruction and its what got us through the cold war alive neither side would face the other head on as they both would be turned to molten slag a few minutes latter you just make clear you have low bar to initiate the Samson option.

as long a they believe you would nuke them for taking a single town they wont do it, so you won't have to.

and wars can be led in such ways that it never seems like the enemy is attacking the thing over which you would be willing to end the world. aka salami tactics

https://m.youtube.com/watch?v=yg-UqIIvang&pp=ygUOc2FsYW1pIHR...