That depends on how far the war extends. If a country thinks they are losing everything, then nukes could be their last resort. I don’t think anyone wants to risk it. The only reason nobody has taken out the lunatic in N. Korea is because he has nukes. It is also why other countries have nukes. Other countries leave them alone, except for the proxy wars in other countries that I mentioned in my previous comment.
Countries don't make decisions. Rulers do. And rulers are remarkably concerned about self-preservation. Even the Gallic chieftain Vercingetorix, a man's man by all accounts, surrendered to Caesar rather than fight to the death. No country is ever going to lose everything except through genocide (including the nuclear variety). Military conquest is not the end. Afghanistan outlasted the Russians despite having a tiny of fraction of Russia's combat power. It's completely irrational to bring down nuclear retaliation on yourself by using a first strike to counter a conventional attack.
The possession, by N. Korea, of a few ungainly, experimental, unreliable, low-yield nuclear devices that are as yet too large to mount on ballistic missiles is not the reason that no one has "taken out" the lunatic ruling that country.
Regards,
It is because he has the full backing of China and/or Russia - usually just one at a time...