Think about it. Would you be willing to end the world if your neighboring country just took one town? How about a city? A region? Countries need more than nukes to defend themselves because it's not credible or sensible to threaten to end the world over what could be just a border dispute.
its not a end of world scenario until you get to superpower vs superpower even then the southern hemisphere would probably hold up fairly well if Russia and NATO went all out.
If you like you can modify the calculus to 'destroy and poison the vast majority of our cities and people' and the calculus holds. Countries need non-nuclear deterrence.
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
The list of wars that countries with nuclear weapons have lost against countries without nuclear weapons is pretty much all the wars those countries have list since 1945. It's a very long list.
On their own ground, deep in the heart of their most populated cities, using their own civilian aircraft, knowing they're unlikely to destroy their own cities and believing that your vision of God is on your side .. sure.
