A true nuclear war would end human life on earth except for a few.pockets.
Why? Let's play more or less realistic scenario. Country A attacks country's B nuclear forces and destroy significant part of them. Then, country A suggest to country B acceptable terms of peace (like just losing few provinces) if country B refuse to retaliate. Country B accept this generous offer, lose few provinces, but continue to exist as a nuclear power.
A massive counter-force strike is a true nuclear war, isn't it?
Unless one rejects the hign probability of a nuclear winter as predicted by.scientists in multiple countries.
Of course, any sane person rejects the nuclear winter conception as an anti-scientific nonsense based on false assumptions:
1. Nuclear war is mostly exchange of nuclear blasts in the cities. It's lie. Both Russia and America prefer to save their citizens to murder civilians of the adversary. Therefore the first strike is counter-force in the both side strategies.
2. Nuclear bursts in the cities will cause massive fires. It's lie. For most air conditions and yields of nukes, radius of demolition is larger than the radius of wood ignition. There will be no "city fires" (as it was in Hiroshima or Tokyo). There will be "soldering in debris".
3. Numerous city fires unite in a super-firestorm. It's lie. In the modern cities with their concrete buildings and wide streets small fires doesn't really spread and unite. You need dense wooden building to have a firestorm.
4. Super-firestorm burn all organic in ash and sooth and send it stratosphere. Its lie. Super-firestorms means high temperature and it means that all organic is burning to CO_2 and H_2O. No ash nor sooth.
5. Ash cause decreasing of temperature. Its lie. The guys simply ignore greenhouse effects of CO_2 and water vapour. And there will be a lot of water vapour and CO_2.
6. The lack of harvest cause starvation. Only if you don't reserves and can't take food from someone else. Winter (nuclear or natural, say, volcanic) doesn't kill people directly. Unpreparedness do. Like, you know, in 1600 volcanic eruption in Peru caused in Russia three "years without summer" (and without harvest). Yes, it caused the fall of Godunov's dynasty, and started "the time of troubles", but most of human losses (30% of Russian population) were caused not by starvation per se, but mostly by another regular European (this time basically Polish) invasion. "The Time of troubles" was ended in 1612, and since then Russian Tzars prefer to be prepared to such things (and keep enough of food supplies for their people).